Autocannon E-Book

Anthony G Williams

• Herausgeber: The Crowood Press
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 1207

Veröffentlichungsjahr: 2022

Beschreibung

Autocannon is a comprehensive history of the development of automatic cannon and their ammunition from the end of the nineteenth century up to the present day. A brief history of their development is followed by sections examining the basic principles of gun and ammunition design, including various unconventional systems. Next comes a survey of cartridges in calibre order, from 20mm to 57mm, including not only ammunition that has seen service, but also a wide range of experimental types. Finally there is a survey of the cannon, grouped by nationality, which again includes many experimental models. The book includes data on about 200 different cartridges and 400 cannon, and has nearly 500 illustrations. It is the culmination of decades of research, and is unmatched in its coverage of this subject.

Leseprobe

57 mm Bofors M/50 twin mounting

First published in 2022 byThe Crowood Press LtdRamsbury, MarlboroughWiltshire SN8 2HR

[email protected]

www.crowood.com

This e-book first published in 2022

© Anthony G. Williams 2022

All rights reserved. This e-book is copyright material and must not be copied, reproduced, transferred, distributed, leased, licensed or publicly performed or used in any way except as specifically permitted in writing by the publishers, as allowed under the terms and conditions under which it was purchased or as strictly permitted by applicable copyright law. Any unauthorised distribution or use of this text may be a direct infringement of the author’s and publisher’s rights, and those responsible may be liable in law accordingly.

British Library Cataloguing-in-Publication DataA catalogue record for this book is available from the British Library.

ISBN 978 1 78500 921 1

Cover design by Blue Sunflower Creative

CONTENTS

ACKNOWLEDGEMENTS

INTRODUCTION

PART I:A BRIEF HISTORY OF AUTOMATIC CANNON

Developments before 1932

1933–1940

1941–1950

1951–1969

1970 Onwards

PART II:DESIGN CONSIDERATIONS

Cartridge Design: Conventional Systems

Ballistics

Gun Elements

Unconventional Systems

PART III:THE CARTRIDGES

20 mm Cartridges

22–28 mm Cartridges

30 mm Calibre

33–37 mm Cartridges

40–57 mm Cartridges

PART IV:THE CANNON

Canada

China

Czechoslovakia & Czech Republic

Denmark

Finland

France

Germany

International

Italy

Japan

Poland

Russia & USSR

South Africa

Spain

Sweden

Switzerland

United Kingdom

United States of America

Other Nations

APPENDICES

Table 1 – The Cartridges

Table 2 – The Cannon

Glossary

Sources

INDEX

ACKNOWLEDGEMENTS

This book could not have been written without the help of the large number of people who have contributed information and illustrations in the twenty years since my first book on this subject – Rapid Fire – was published. I am very grateful to all of them.

Many of them have sent material to the ECRA Bulletin (European Cartridge Research Association) of which I was the editor for fourteen years. Others have been active on specialist internet forums, especially the IAA (International Ammunition Association), BOCN (British Ordnance Collectors Network), and my own forum, Military Guns and Ammunition. The web addresses are included in the Sources section at the end of the book, along with those of other sites which have provided useful information and a small selection of the books and other documents I have learned from. My education in this subject has also benefited from thirteen years spent as an editor of the Jane’s ammunition reference, which has been through various name changes and is currently Jane’s Weapons: Ammunition.

Crediting all of the people who have helped me would be an impossible task, but I would like to name several individuals whose help was particularly significant, or have reviewed and/or made major contributions to sections of this book, as follows:

General

Richard Jones, formerly of the Ministry of Defence Pattern Room and the National Firearms Centre

Steve McGregor, ECRA

And some who are no longer with us, including: Peter Labbett (ECRA), William Woodin (IAA) and Herbert Woodend (MoD Pattern Room)

Ammunition feeding arrangements (magazines, belt links, clips etc)

Koen Bakker

The following national sections

France:

Claude Jacquinet

Germany:

Alex Diehl

Italy:

Viligelmo Ballabio

Japan:

Yasufumi Kunimoto

Russia:

Evgeny Aranov, plus Maxim Popenker and Denis Estavyev

Sweden:

Sigbjörn Hellström

Switzerland:

Robert Gerber

UK:

Tim Gurney

USA:

Robert Leiendecker

INTRODUCTION

The term ‘cannon’ in British use originally referred to large-calibre muzzle-loading artillery. This book is concerned with modern cannon; while there is no agreed single definition, these are commonly regarded as being fast-firing high-velocity automatic guns with calibres (projectile diameters) between 20 mm and about 57 mm.

The precursors of such guns were developed in the late 19th century, in the form of larger versions of hand-cranked multi-barrel machine guns (MGs) like the Gatling and the Nordenfelt, which were made in calibres of up to one inch (25.4 mm) and fired solid projectiles. These were followed by the Hotchkiss cannon revolver with a rotating cluster of five barrels, also hand-cranked but in much larger calibres of 37 mm, 40 mm, 47 mm and 53 mm, which fired hollow shells filled with explosives.

2-barrel 1-inch Nordenfelt (4 barrel versions also saw service)

37 mm Hotchkiss ‘cannon revolver’

The first truly automatic cannon came near the end of that century, in the form of the Vickers-Maxim One Pounder, so-called from the nominal weight of the projectile (1 lb or 454 g). This was a scaled-up version of the company’s machine gun and used the recoil energy from firing each shot to power the firing cycle. This famous gun was nicknamed the ‘pom-pom’ from the distinctive sound of its firing, a designation which was also applied to its successors, particularly the British 2-pounder naval anti-aircraft (AA) gun of World War II.

World War I saw the first extensive use of cannon in a variety of roles, most notably against the growing threat of aircraft attacking ground or naval targets. Some were also mounted on aircraft, for firing at each other or at ground targets. The guns were still in their infancy at this stage and developed only slowly in the interwar years. Only in the late 1930s was there an accelerating rush of interest in acquiring such weapons, and World War II saw such famous names as Oerlikon, Bofors and Hispano become synonymous with cannon, principally for use against, or by, aircraft.

After World War II the development of cannon continued for a variety of roles, but from the late 1950s they became increasingly challenged by guided missiles. However, experience revealed that the theoretical performance of the early missiles was rarely achieved in combat, so cannon remained in use as primary weapon systems against aircraft for longer than expected. Cannon also found other roles, particularly for use by and against light armoured vehicles; their principal use today. Their versatility has also led to their continued use: by combat aircraft (as a back-up weapon, or for use against targets for which a missile is not appropriate); in the ground-based anti-aircraft role (to supplement missiles); and also installed on warships, for dealing with small boats and/or as a last-ditch defence against incoming anti-ship missiles, in which role they are known as Close-in Weapon Systems (CIWS). The latest application of cannon is to adapt the naval CIWS to the C-RAM role (Counter-Rocket, Artillery and Mortar), for the protection of military land bases.

Alternative weapons keep emerging, in the form of smaller and cheaper missiles or entirely different technologies such as lasers, but it is likely that there will be roles for cannon for the foreseeable future.

GUN CALIBRE

The choice of the optimum calibre for any particular purpose is inevitably the result of various compromises. Other things being equal (an important qualifier in any statements comparing guns and ammunition), large calibre weapons provide greater destructive power and a longer effective range, but are bigger and heavier, generate more recoil, have a lower rate of fire (RoF), and the bigger ammunition restricts magazine capacity.

The popularity of different calibres has varied depending on the tactical needs of the time. However, the first true automatic cannon emerged in the nineteenth century in 37 mm calibre for legal reasons. In the middle of that century explosive projectiles were introduced in rifles. These were felt to be inhumane when used against soldiers, so an international convention was held that led to the St. Petersburg Declaration of 1868, which stated:

The Great Powers agreed to renounce, in case of war among themselves, the use of any explosive projectile of less weight than 400 grams (14 ounces avoirdupois) or one charged with fulminating or inflammable substances.

At that time, a 400 g shell was typically of 37 mm calibre so this became an international standard which was honoured for the next half century. By the start of World War I, except for a few obsolete weapons there was a huge calibre gap between rifle-calibre (6.5 to 8 mm) MGs and 37 mm cannon.

The pressure of war eventually led to a demand for smaller cannon calibres, partly for the anti-tank (AT) and anti-aircraft roles and partly to provide more firepower for aircraft without burdening them with large cannon. Aircraft weapons in 20 and 25 mm were developed which, although initially available only with solid shot, used high explosive (HE) projectiles by the end of the war.

After World War I was over, it was decided that the legal position of explosive shells should be clarified so in 1922 a Committee of International Jurists met at the Hague to consider a set of rules for aerial warfare. They decided that the use of tracer, incendiary or explosive projectiles by or against aircraft was not prohibited. This draft Convention was never formally ratified, but everyone thereafter behaved as if it had been, albeit with a degree of nervousness in some quarters.

In the interwar period, 20 mm became generally regarded as the smallest calibre whose projectiles were capable of carrying a useful quantity of HE. It therefore became a standard calibre with the great majority of nations and has remained so up to the present time. The 37 mm calibre remained popular and was joined by 40 mm weapons from Britain and, most notably, Bofors in Sweden. This remained the largest calibre to see significant service in rapid-firing weapons until the middle of the 20th century, with the exception of the early Hotchkiss manually-cranked ‘canon revolver’ (actually a rotary gun in modern terminology), produced in 47 mm and 53 mm calibres.

Before World War II there were few weapons in calibres intermediate between 20 and 37 mm, the most significant being 25 mm AA guns used by Sweden, France, Japan, Russia and, just in time for the war, the American 1.1-inch (28 mm) naval gun (whose odd calibre indicates that this was probably the last cannon to have been designed to adhere to the St Petersburg Declaration, since the shell weight was 416 g). More emerged during the war, with 23 mm weapons becoming established in the Soviet Union and 30 mm guns, initially for aircraft, in Germany, with Japan following suit right at the end. Attempts were made in various countries to produce fast-firing automatic weapons in 50–57 mm calibre, but cannon of this size were not notably successful until after the war.

After the lessons of the war had been absorbed, NATO broadly settled for automatic cannon in 20, 30 and 40 mm calibre (with 35 mm, 27 mm and 25 mm being added later) plus 57 mm in naval guns (land and aircraft versions not lasting long). NATO cartridges are allocated STANAG (STANdardisation AGreement) numbers, which specify characteristics required to ensure inter-operability in different weapons. The Warsaw Pact (and by extension China and nations supplied by these countries), used weapons in 23, 25, 30, 37, 45 and 57 mm calibres. It should be noted that NATO countries currently produce or use weapons chambered for three different 20 mm cartridges, two 25 mm and four 30 mm (not including variations using different case materials or priming) as well as 27 mm, 35 mm, and soon to be two, or even three, 40 mm, and a 57 mm. In contrast, the complete and considerable range of Russian automatic cannon currently in production is focused on the 30 × 165 cartridge (albeit in both percussion and electrically-primed versions), although the 57 mm seems to be enjoying a minor revival of interest.

DEFINITIONS

The current usage of the term ‘cannon’ has only become widely accepted since World War II. Before then, the term ‘machine gun’ was used to describe a relatively small-calibre weapon normally firing solid projectiles while larger-calibre versions were generally called ‘automatic guns’. The situation changed in British practice with the selection for the Royal Air Force (RAF) of the French Hispano Suiza HS 404 moteur-canon in the late 1930s. Anglicized as ‘cannon’, the name became adopted for the Hispano and subsequently for all other automatic shell-firing guns of 20 mm or more calibre. The plural of cannon can be either cannon or cannons – the former will be used in this book.

The distinction between MGs and cannon is an arbitrary one that has varied in different countries and at different times. As far as projectiles are concerned, the accepted convention has been that MGs are designed to fire bullets: ball, incendiary and armour piercing (AP) projectiles, with or without tracers; that are usually ‘jacketed’; the core being covered by a relatively soft, usually copper-based, alloy which can be formed by the pressure of firing to fit the rifling grooves. Cannon projectiles – generally known as shells – are normally of steel with separate, softer driving or rotating bands to take the rifling, and are typically filled with HE and/or incendiary materials plus a fuse to initiate them, or have a hard metal core for armour piercing (AP shot). The dividing line between bullets and shells is generally accepted to be 20 mm calibre, exceptions to this general rule being the 13 mm Rheinmetall-Borsig MG 131 and 15 mm Mauser MG 151 aircraft heavy machine gun (HMG) rounds, which used cannon-type projectiles; conversely, the AP shot for the 25 mm Hotchkiss cannon was made just like an MG bullet.

Different nations have had different naming practices. For example, in World War II German automatic weapons of up to and including 20 mm were known as machine guns (Maschinengewehr) with larger calibres being known as cannon (Maschinenkanonen), leading to the MG and MK prefixes. In addition, designations based on function were used for particular applications, whether or not the weapons were automatic. A gun intended for the anti-aircraft role was called Fliegerabwehrkanone (sometimes given as Flugzeugabwehrkanone or Flugabwehrkanone), for the anti-tank role Panzerabwehrkanone, and for mounting in armoured fighting vehicles Kampfwagenkanone; Flak, Pak and KwK for short. The BK (Bordkanone) designation was used for very large calibre (37+ mm) airborne cannon intended for ground attack, although currently that term is applied to the 27 mm Mauser BK 27 cannon.

Nomenclature also varied between services. In Sweden, the Air Force called their 12.7 mm and 13.2 mm m/39 guns automatkanon, presumably because they were larger than usual for aircraft guns at the time, while their Navy called the 25 mm M/32 a kulspruta, (which translates as machine gun), presumably because this was a small weapon by naval standards. In the US services, naval weapons are generally given Mk (mark) designations, army guns M (model). The numbers that form part of such designations have in most cases related to the date of introduction or acceptance into service (e.g. M/32 introduced in 1932) but in other cases model numbers are allocated on a different and not always obvious basis, as is typical of current US and UK practice.

Since World War II the definition of a cannon as any fast-firing, high-velocity automatic weapon of 20 mm or more has become commonly accepted; the upper calibre limit is less clearly defined, but is generally taken to be around 57 mm as, broadly speaking, this is the largest calibre of automatic weapon to include multi-purpose guns that were fitted to aircraft, naval and land mountings. The next step up in automatic weapon calibre is to 75 mm, which are usually specialist single-purpose guns.

Calibre designations have also varied. The UK and related countries used to designate guns of 37+ mm calibre (whether automatic or not) by ‘pounder’ measurements (abbreviated to Pr or Pdr, often written PR or PDR), after the nominal weight of the standard projectile in pounds. The British also used inch measurements, but by the 20th century mostly for guns from 76.2 mm (3-inch) calibre upwards. The USA used inch measurements as standard for some weapons (e.g. the 1.1-inch naval gun) but from World War I onwards also used metric measurements, having adopted 37 mm, 75 mm, 105 mm and 155 mm artillery from France during that war. Only in the last half-century have metric calibre measurements almost entirely taken over from inch or pounder measurements in Britain and the USA. Even within the metric group, practices have varied; in wartime Germany and certain other countries, heavy weapon calibres of 20 mm or more were usually described in centimetres rather than millimetres.

The cartridges fired by the guns are also described using what has become accepted as the standard metric measurements of calibre and cartridge case length, plus any non-metric designations they have been known by. The metric measurements may not have been used at the time (e.g. the USN 1.1-inch cartridge was only ever known by that name; the metric 28 × 199 SR is a retrospective designation for comparison purposes). Metric measurements are therefore used for all calibres and cartridges in this book, alongside any other designations which may have been used. In some instances two or more cartridges may have the same measurements, so these are distinguished by additional letters which may refer to the rim design (R, SR and RB or RR for rimmed, semi-rimmed or semi-rimless, and rebated – with a small-diameter rim) or some other identifying letters, notably B to describe a belted case. The worst case of this lies with 20 mm cartridges with 110 mm case lengths: six in total (including experimentals), none of which are interchangable. So as well as 20 × 110 RB and 20 × 110 R there are three named for the gun: 20 × 110 HS (Hispano Suiza), 20 × 110 Scotti, 20 × 110 AP-20; and one named for the user organisation, the 20 × 110 USN. Again, these designations may not have been officially used, but are applied now to distinguish between them. A third metric to help with identification is the rim diameter; this is shown in the ammunition tables.

A final note on ammunition designations: in larger-calibre artillery in which the projectile and propellant are loaded separately, a ‘round’ of ammmunition refers to the projectile, the ‘cartridge’ is the separate propellant charge. However, in small arms and cannon, in which the projectile is firmly secured to the cartridge case during assembly, the complete unit of ammunition – cartridge case, propellant, projectile and primer (means of igniting the propellant) – may be known as either the ‘cartridge’ or the ‘round’ of ammunition.

BOOK STRUCTURE

This work is structured into four parts: first, a brief history of cannon and their use; second, the design of cartridges and guns; third, the cartridges described in calibre order; and fourth, the cannon arranged by country. Finally there are data tables on the cartridges and guns for quick reference purposes, and a glossary of terms.

Part I provides a history and analysis of the use of automatic cannon by armies, navies and air forces of all nations from the end of the 19th century to the present day. It is focused mainly on those guns which made it into service, and describes how their use changed over time.

Part II considers the main factors affecting the design of cartridges and cannon, including the different types of projectile used, a brief section on ballistics and consideration of unconventional systems.

Part III includes all of the cannon cartridges to have seen service, plus many which did not.

Part IV gives the same treatment to the cannon.

It should be noted that the focus in this book is sharply on the guns and ammunition, with some information about mountings but nothing about sights or fire control. These omissions are not due to any disregard of their importance – in fact, they are vital to the effective performance of the weapon systems – but they are complex subjects of their own and deserve a much more detailed treatment than there was space for in this book.

PART I: A BRIEF HISTORY OF AUTOMATIC CANNON

DEVELOPMENTS BEFORE 1932

The late 19th century was a period of rapid technological development in guns, dramatically increasing their volume of fire. The key to this was the introduction of the self-contained cartridge or round of ammunition, loaded with ‘smokeless powder’ which did not foul up mechanisms in the same way as black powder (gunpowder), and which was sturdy enough to survive the violent handling needed to achieve high rates of fire. The precision engineering needed to mass-produce the many small mechanical parts of a fast-firing gun also developed.

The first self-powered machine gun was the Maxim, which entered service in 1886. This used the recoil force from firing each cartridge to push the breechblock back, extracting and ejecting the fired cartridge case then removing the next round from a fabric belt before pushing it into the chamber (‘chambering’ it) to be fired.

The Maxim machine gun was adopted by many armies, usually firing the same ammunition as the army’s rifle. However, interest grew in a bigger model to compete with the hand-cranked multi-barrel Hotchkiss revolving cannon. Maxim selected the smallest of the Hotchkiss 37 mm rounds, the 37 × 94 R, as the basis of their large machine gun.

German Maxim 37 mm captured in southern Africa (NJR ZA – Wikimedia Commons CC BY-SA 3.0)

Designed in the late 1880s, the first versions entered service in the 1890s and the gun was adopted by the British Army in 1900 as the ‘Ordnance QF 1 pounder Mk. 1’ (QF stood for Quick Firing, an artillery term which meant that the propellant was contained within a brass cartridge case). However, it was popularly known as the ‘pom-pom’. The explosive shells were not only valued for their destructive effects, but also because at long range the puffs of smoke indicated where the shells were landing, allowing the aim to be corrected as required.

The pom-poms were widely used by both sides throughout World War I, although they became obsolete in ground and naval fighting. In ground fighting, they were outranged by modern fast-firing artillery, and in naval use they were no longer effective against larger torpedo boats and torpedo-boat destroyers. However, when installed on high-angle mountings they found a new role against low-flying aircraft. The first installation of a pom-pom on a high-angle mounting for anti-aircraft (AA) fire was actually approved for the British Army before World War I, in February 1914.

Naval 2-pounder pom-pom (Wikimedia Commons, public domain)

Maxim’s basic recoil-operated cannon design was scaled up and down, and modified to accommodate the use of different cartridges from the Vickers 1-inch gun (25.4 mm) to the Vickers 2-pounder (40 mm) naval AA gun, as well as versions retaining the 37 mm calibre but with smaller or larger cartridge cases to meet specific needs. In World War I Britain briefly made use of the 37 mm Vickers 1½-pounder installed in warships for the AA role (some also equipping the Austro-Hungarian navy), quickly replaced by the 2-pounder; while the small 1-pounder Mk III aircraft gun also saw very limited use. The USA developed the One Pounder ‘Heavy’ 37 mm Mk. 6 naval gun, while Krupp produced the 3.7 cm Luftschiff Flak gun, better known as the Sockelflak.

Not all such guns were inspired by Maxim’s design. In France, Hotchkiss produced Le canon automatique de 37 mm Hotchkiss Mle 1900/1901, an enlarged version of the company’s gas-operated machine gun, and this achieved some sales. Gas operation was also adopted by Samuel McClean in the USA for his 37 mm cannon which was first tested in 1904. The gun proved too unreliable to pass US tests but a couple of hundred were sold to Russia during World War I and turned up in many places thereafter.

Long recoil was the method of operation chosen for the British 1½-pounder C.O.W. gun (named after the Coventry Ordnance Works where it was developed). This gun used powerful ammunition, but only just got into service at the end of World War I and, after various attempts to arm aircraft with it (some aircraft were even designed around it, but did not see service) the surviving guns saw their last use in the AA role in World War II.

The need to ignite the hydrogen which provided the lift for observation balloons and airships led to the introduction of explosive/incendiary bullets even in MGs, which were clearly in breach of the St Petersburg Declaration (described in the Introduction). However, after World War I lawyers argued that the purpose of the Declaration was to prevent explosive bullets being used against personnel, and using them solely against aerial craft of any type complied with the spirit if not the letter of the agreement. This argument was generally accepted thereafter.

High-Explosive/Incendiary (HEI) MG bullets were really too small to be effective, so interest grew in smaller cannon, firing shells large enough to hold a useful quantity of HEI. The 1-inch Vickers (made in the Italian Terni works) and the similar 25.4 mm Revelli-FIAT did fire HEI ammunition at least later, if not initially, but saw little use in World War I, mostly flexibly-mounted in Italian bomber aircraft. In the early 1920s the Vickers-Terni was sold to Sweden for arming submarines and remained in service to the late 1930s.

Much more significant was the German 20 mm Becker, which was first designed and tested just before World War I. The Becker had a unique operating system, now called Advanced Primer Ignition (API) Blowback. It was very simple, with few moving parts, and had the effect of softening the recoil blow, so was particularly suitable for use in aircraft. Germany made several hundred Beckers during World War I, about half arming aircraft, the others on ground mountings for both AA and anti-tank (AT) use.

2 cm Becker (left) with vertical box magazine, alongside a belt-fed 7.9 mm MG 14 Parabellum (Courtesy Philip Jarrett)

After the War, Germany was prohibited from making such armaments, so further development was transferred to Switzerland and undertaken first by SEMAG and later by Oerlikon. By the mid-1930s these guns, in three versions (all 20 mm calibre but with three sizes of cartridge: 20 × 72 RB, 20 × 101 RB and 20 × 110 RB) were thoroughly developed and available on the open market. However, relatively little attention was paid to them until World War II created a huge demand for aircraft and AA cannon.

Oerlikon was not the only manufacturer developing cannon in the early interwar period. Rheinmetall of Germany nominally transferred its prohibited cannon development work to its Swiss subsidiary, Solothurn. This was based on the experimental Ehrhardt recoil-operated 20 mm gun of World War I, and while the 20 mm calibre was retained, the cartridge size was substantially increased, from 20 × 70 RB to 20 × 105 B and 20 × 138 B. However, very few sales initially resulted.

The other arms manufacturer offering 20 mm cannon at this time was the Danish company, Madsen. They had made their name with a light MG early in the century, which had many sales worldwide and remained in production until at least the 1950s, but was never adopted by a major army. Much the same could be said of their 20 mm cannon, which like many early guns of this type was marketed as a dual-purpose weapon: for AT purposes on a low mounting and AA on a high-angle one. With a 20 × 120 cartridge it was more powerful than most, and a cyclic rate of fire (RoF) of 400 rpm was par for the course at that time. It saw service with various armed forces, was still being marketed by the company in the 1950s, and ammunition was still being made for decades thereafter.

1933–1940

In 1933 automatic cannon were still a rarity in service. Most armies didn’t see the point of them, most aircraft struggled to carry them, and only navies had really identified a clear need for them. They were very aware of the threat to their ships posed by bomb and torpedo-carrying aircraft and wanted to be able to shoot them down before they could release their weapons. That meant a layered gun defence, with manually loaded guns of 76 to 127 mm calibre (3 to 5 inch) as the outer layer, something in the 25 to 40 mm calibre range (preferably in multi-gun mountings) for the middle layer, and MGs (initially) or 20 mm cannon (eventually) mainly on unpowered mountings as the inner layer.

NAVAL AA

The principal issues in selecting naval guns for the middle layer of defence at this time were: the calibre (range versus volume of fire); the choice between a high-velocity manually-loaded gun or a lower-performing automatic; the ship impact of the mounting – size and weight (linked to the number of barrels required per mounting); what kind of ammunition feeds were more suitable for extended engagements; and whether barrel cooling, power operation or even stabilisation of the mounting, were worth the added complication.

It should be noted that the tactical situations of navies required different solutions from the armies: navies expected their warships to be subjected to successive waves of air attacks, putting a premium on the ability to sustain continuous fire for an extended period, something which required water cooling or very quick-change barrels. On the other hand, weight was generally less critical aboard ship, so large, multi-barrel mountings were attractive, particularly when coupled to more sophisticated sighting systems.

The method of aiming was an important factor; MGs and light cannon used open sights so their accuracy was greatly dependent on the gunner’s skill. The larger automatic cannon mountings commonly featured director control, which added to the mounting and manning demands.

Powered mountings offered much faster training and elevation rates and (if coupled with stabilisation to compensate for ship rolling and pitching) greater accuracy, but were more expensive and vulnerable to loss of power through action damage.

Also of concern was the crew required to man the weapons (as warships were commonly overcrowded in wartime) and the maintainability of the equipment; World War II experience led to simplicity and reliability becoming highly valued.

The most prominent cannon, in terms of service with a major nation, was World War I-vintage British 2-pounder pom-pom naval AA weapon in a single mounting, nearly 800 being made for the RN, and many export sales being achieved. The pom-pom was unusually advanced for the time in having a belt feed for the ammunition (initially fabric), plus a water-cooled barrel to sustain a high rate of fire. This was supplemented by a modified gun in an eight-barrel (and subsequently four-barrel) mounting from 1930 but production was initially very slow. Furthermore, even as the octuple mounting began to enter service there was some concern about its adequacy. However, an attempt to replace the 2-pounder with a more modern system failed; instead of this, a high-velocity loading of the pom-pom was developed.

Quad Pom Pom mounting, showing the steel-link ammunition belts used by the later Marks (RAN)

One other naval AA cannon was developed in Britain in the 1930s: the high-velocity Vickers 25.4 mm gun. The only user was Argentina, and the only ship it was fitted to was a training cruiser launched in 1938, so it was of no practical significance.

Italy took the needs of the navy more seriously than most, the 37 mm Breda 37/54 being an automatic weapon available from the start in a M32 twin mounting with water-cooled barrels (although later models were air-cooled) and, very unusually for a system in this class, a limited stabilisation system.

The USN was slow to introduce an equivalent system; the 1.1’ Mk 1/1 gun was first tested in 1933 but was not available in quantity until 1940. The system proved to require expert maintenance, the rate of fire was low for the calibre, and the projectiles were also rather small for a gun in this class, so it was rapidly eclipsed in popularity by the 40 mm Bofors as soon as that became available.

Two quad 1.1-inch mountings on the USS Enterprise (USN)

The other major navy to select a calibre smaller than 37 mm for its mid-layer defence was Japan, which bought the 25 mm Hotchkiss to replace the Vickers 2-pounder. The gun was designated the Type 96 (indicating that it entered service in 1936) and was fitted into single, twin and triple mountings. Vast numbers were made, but there were criticisms: the air-cooling and magazine-feeding greatly limited the sustained rate of fire, the calibre was rather small, and there were problems with vibration, slow training and elevation rates, and inadequate sights.

This view of a triple mounting for the Japanese 25 mm Type 96 AA gun (originally naval, this one in land service on Guadacanal, October 1942) shows the three vertically-mounted 15-round box magazines (USMC Photo)

Ironically, the French navy did not order the 25 mm Hotchkiss until too late (although a few were fitted to French ships after the German invasion). Their existing systems, the 37 mm Mle 1925 (single barrel) and Mle 1933 (twin barrel), used manually-loaded air-cooled guns firing high-velocity ammunition, as did the German 37 mm SKC/30 and the Russian 45 mm 21-K; all were limited to firing at about 30 rpm. These nations all attempted to replace their systems with automatic cannon, with varying results. France held a competition which resulted in the 37 mm Mle 1935 being chosen, in a twin mounting, but it was still at the trial stage when Germany invaded in 1940. Germany developed the little-known but apparently Bofors-inspired 3,7 cm SK C/36, but few were built. Its unique ammunition as well as general design survived to be used in the Flak M42, which only entered service late in the war, followed by the Flak M43, which used the same gun and ammunition as the army Flak 43. Only the Russian navy did the obvious thing from the start and fitted the 37 mm M1939 developed for the army, which served alongside the 45 mm guns from 1941. However, these army-pattern guns relied on air-cooling, limiting their sustained firing rates.

The Bofors factory in 1936, showing the production line for naval 40 mm twin mountings, with water-cooled barrels

This brings us to the most successful of the medium-calibre cannon developed in this period: the 40 mm Bofors. The first model was introduced in 1932 but sales didn’t really get going until the middle of the decade with the M36 twin naval mounting. The navies who were the biggest users of the Bofors in World War II were the RN and the USN, but production was initially unable to keep up with demand so they did not get the guns into service until the 1941–50 period covered in the next section. Bofors also produced guns in 25 mm and 20 mm calibre in this period, but they were nowhere near as successful. The 25 mm M/32 did see service in the Swedish navy and also inspired the development of a Russian naval system in this calibre, introduced later in the war. The 20 mm m/40 was mainly used by the army.

Now to the inner-layer naval systems, or ‘last ditch defence’! The MGs fitted prewar proved inadequate in this role, being steadily replaced with shell-firing cannon of 20 mm calibre, almost invariably in a simple, free-swinging, single-barrel unpowered mounting with open sights.

Barrels were air-cooled, and magazine feed was almost universal. Several models were available, from Madsen, Breda, Lahti, and Rheinmetall, but by far the most popular was the largest and most powerful of the Oerlikons, the Type SS, which was made and used in vast numbers by both the RN and USN.

The 20 mm Rheinmetall mounting was the L/30, using their C/30 gun (the same as the land-based Flak 30), which weighed 420 kg; this became the L/38 when fitted with the Mauser C/38 gun, which first appeared at the very end of this period. Italy used the same Breda 20/65 M35 as the army, in various mountings including a complicated twin in a diagonal mounting (the RM35, featuring power operation and limited stabilisation, and weighing 2,330 kg) in which one gun was above and to one side of the other, plus two single mountings; the little-used M39 and the free-swinging M40 of 312 kg.

The gun crew of a 20 mm Oerlikon aboard a Greek warship in the Mediterranean (Wikimedia Commons / public domain)

The multi-purpose Madsen cannon saw some use on naval mountings, and a few of the Finnish Lahti L34 were fitted to small naval craft.

Some use was also made of the Hispano aircraft cannon in this role, but it was not a success. The USN BuOrd (Bureau of Ordnance) history, as quoted in Friedman’s ‘US Naval Weapons’, recounts the following concerning the Oerlikon and Hispano as naval AA guns:

‘…the Oerlikon offered greater reliability because of [its] greater reserve of power. It was operated by maximum gas pressure, about 23 tons per square inch, and by springs which were correspondingly powerful. On the other hand the Hispano-Suiza was operated by reduced gas pressure of about 8 tons per square inch and by springs which were similarly weak… the forces at work in the Oerlikon were consequently greater than those in the Hispano, and hence such factors as friction, the effect of different elevations, cold weather, rain and the like, were too small in proportion to the operating forces to cause stoppage. Primarily as a result of this reserve power, the Oerlikon did not require skilled service maintenance. One of the greatest advantages of the Oerlikon… was [that] it had a barrel which could be replaced in about 30 seconds. Since an airplane gun fired very short bursts, and was mounted in the wings of a plane moving through the air at a high rate of speed, it did not present much of a cooling problem. The Hispano was therefore designed without any thought of permitting a change of barrels during action; in fact, it was hardly possible under such conditions. At sea, however, against multi-plane dive-bombing attacks, prolonged firing was necessary… The magazine employed with the Oerlikon was superior… the Oerlikon magazine could be kept fully loaded without any tension on the springs while in the Hispano the spring was compressed as long as the magazine was loaded, a condition which brought about occasional spring collapse. Secondly, the magazine had a greater reserve because of the easier task of feeding at 490 rpm as against the Hispano’s 690. Thirdly, the Oerlikon magazine had a straight tangential lead through its mouthpiece, whereas the Hispano had a 90 degree bend…’

Thenavalexperience of the British with the Hispano-Suiza proved the superiority of the Oerlikon. After the fall of France supplies of Oerlikons from Switzerland were cut off, and as a stop-gap Hispano guns were installed on several escort vessels. The results were uniformly bad. Ships reported frequent stoppages from the unsatisfactory design of the cradle, salt water corrosion around the breech block, unlocked tappets, and the collapse of the magazine springs… when water got into a hot gun barrel stoppage was likely at the reopening of fire. The guns were so unreliable that one vessel reported that she preferred not to open fire with her Hispanos in the hope that the Germans would think that she was not a warship. Opposed to this, British experience with the 100 Oerlikons at sea in November of 1940 was uniformly excellent.

LAND AA

Most armies showed very little interest in AA cannon until the end of the 1933–1940 period. The typical approach was to field manually-loaded 75+ mm guns, backed-up by MGs for close defence. What changed this attitude was the Spanish Civil War (1936–39), an intense conflict in which several countries became involved, mainly for ideological reasons but they also took the opportunity to test how their latest weapon systems and tactics worked in practice. The Republicans (loyal to the existing democratic government) were supported by Russia, while the eventually successful Nationalists led by General Franco were supported by Germany and Italy. France, the UK and the USA stayed officially neutral but their militaries observed developments with professional interest.

One of the lessons of this conflict was the devastating potential of air attacks and the inadequacy of existing AA systems to deal with them: the 75+ mm guns were too slow-firing, and their elevation and training rates too low, to engage fast-moving planes, particularly at low level, while MGs were not destructive enough. The result was a sudden interest in cannon for AA purposes. The initial focus was primarily on medium-calibre guns (34 to 40 mm) which were in many cases similar to those being sought for middle-layer naval defence. However, the requirements of land forces were somewhat different from the naval ones. Mobility was far more important, so minimising weight was a priority and single-gun mountings with air-cooled barrels plus manual training and elevation were the norm. The lack of sustained fire capability which resulted was not regarded as much of a problem, as it was expected that air attacks on ground forces would be more sporadic and dispersed than attacks on ships, with engagement opportunities for the guns being brief and widely separated.

The British Army initially took the logical step of adopting a version of the naval 2-pounder pom-pom, but when the 40 mm Bofors gun was evaluated in 1937 its superiority in terms of mobility, ballistics and reliability was so obvious that the Army changed tack and ordered the Bofors instead, the few 2-pounder which had been delivered being used for static defence before ultimately being handed to the RN. Between 1937 and 1939, orders for more than 500 guns were placed with Bofors, with licensed domestic production beginning in mid-1939. The typical towed Bofors mounting for land use weighed around 2,000–2,500 kg.

40 mm Bofors in Dutch Indonesia, post-World War II

The US Army was the other major, large-scale user of the Bofors (although many other armies bought it in smaller quantities), but they used it alongside their Browning-action 37 mm M1.

Russia used the 37 mm M1939, derived from a Bofors design. Germany had the benefit of Rheinmetall’s considerable resources and experience, so they had the 37 mm Flak 18 cannon in service in 1935; this was rapidly replaced by the Flak 36 and then the 37, but the changes concerned the mounting and sights, not the gun. Italy had the Breda 37/54. All of these guns had a comparable performance in terms of ballistics and rates of fire, and all of them used magazine or ammunition strips for feeding, but the quality of their mountings and sights (crucial issues in determining effectiveness) is more difficult to evaluate.

One of the earliest army AA cannon to be developed was the French Schneider 37 mm Mle 1930, using its own unique and quite powerful ammunition. Interest at this time was negligible, however, and the gun did not achieve any significant sales until the French Army realised that war was looking increasingly imminent, and their preferred 40 mm Bofors would not be available for some time. An interim order for the Schneider was placed, but only a handful of guns made it into service. A 37 mm Hotchkiss saw even less use.

A very interesting system in this class was the Swiss 34 mm Flab K38. This was unusual in two respects (in addition to its calibre): it had belt feeding and a water-cooled barrel, thereby enabling a much higher sustained rate of fire than most other guns in this class. Even the cyclic rate was much superior to other pre-war systems in this class, as it fired at 250–270 rpm instead of the more usual 120–160 rpm. Only the Swiss acquired it, but it stayed in service until the late 1960s.

This takes us the ‘inner layer’ cannon, which were of 20 to 25 mm calibre. Before the fall of France, the 25 mm guns in service were mainly confined to a couple of hundred M1940 twin Hotchkiss mountings with the French army. The Russian army adopted the Bofors-based 25 mm M1940 which entered service in the following year.

The 20 mm guns were generally marketed as dual-purpose AA and AT guns (in the 1930s the AP ammunition of many of them could penetrate most tanks). This was of course dependent on the mounting, a high-angle mount being needed for the AA role, a low tripod for AT; some were convertible between the two. All cannon were air-cooled. The great majority of the guns were fed by magazines: either box magazines typically of 20 rounds (mounted horizontally or vertically) or drums (typically of 60 rounds, but both smaller and larger were used).

Some used strip-feed, in which clips of cartridges, held together at the base by metal strips, were placed on trays feeding into the side of the gun (permitting continuous feed without the need to pause for a magazine change – but the ammunition was exposed to the elements instead of being protected). Only a small minority were belt-fed.

First in the field were Oerlikon, with variants of their Type L and Type S; photos exist of an anti-tank model on a tracked trailer, for towing behind a small tracked vehicle. Madsen were marketing their 20 mm Model 1935 with some success, and in the German invasion of 1940 the Danish army knocked out several light AFVs with them. The Swedish army adopted the Bofors 20 mm m/40. The Swiss had their 20 mm Flab K38 which again differed from most other guns in this class in using a belt feed rather than magazines.

Oerlikon 20 mm gun on an interesting anti-tank mounting

In Finland, Lahti introduced the M40 but the designation is somewhat misleading: the prototype was commissioned in 1939 but due to the lack of German Flakvisier sights, it did not get into service until much later. Italy had two 20 mm AA cannon from different designers: the Breda 20/65 M35 came first, followed in 1938 by the Scotti 20/77. These two guns, and the Lahti, all used Rheinmetall’s 20 × 138B ammunition.

Rheinmetall put a great deal of effort into developing effective 20 mm cannon, initially marketed through their Swiss subsidiary, Solothurn, as a result of treaty limitations on German arms production following World War I. The first into service was the Flak 30, in 1935. From 1940 this was supplemented in service by a modified version by Mauser, the Flak 38, which had a higher rate of fire (420–480 rpm rather than 280 rpm). There was a lightweight mounting for mountain and airborne troops (the 2 cm Gebirgs Flak) and a successful quad mounting, the Flakvierling, much feared by Allied close-support fighter-bombers. Different versions of these cannon (the KwK 30 and 38) were mounted as light AFV armament. All of these were fed 20 × 138 B ammunition via horizontal magazines.

20 mm Madsen in the AT role; a 40-round drum magazine is fitted, belt-feed was available

The Imperial Japanese Army (IJA) adopted 20 mm as their standard light AA cannon calibre. Their first attempt was a massive 20 × 157 cartridge in a modified Rheinmetall/Solothurn gun designated Type 94, but this was unsuccessful so quickly replaced by the Type 98, firing 20 × 143 ammunition. Despite an effort to copy the Bofors they never did manage to get a larger-calibre AA cannon into service, although later in the war they acquired some 25 mm Type 96 mountings from the navy.

Poland and Russia, as well as Germany, mounted 20 mm cannon in some of their light AFVs for ground fighting.

AIRCRAFT CANNON

The French built on their World War I experience of manually-loaded 37 mm calibre moteur canon, this time focusing on 20 mm cannon. One benefit of such a centrally-mounted position was that the projectile trajectory remained in line with the sights at all ranges, without the lateral harmonisation issue of wing-mounted guns (although the sights still needed to be adjusted for vertical harmonisation at some specified range). Other advantages of moteur canon were that the weight was concentrated along the centreline rather than spread along the wings, which helped with the aircraft’s roll-rate; the gun could be rigidly attached to the crank case to help with recoil control (some strengthening of the crank case was required); there was likely to be room for a bigger magazine in the fuselage; and (in some cases) it permitted the use of a bigger gun than would be desirable in the wings. Cowling or wing-root mounted guns also enjoyed some of these benefits, but they needed to be synchronized with propeller rotation to avoid the projectiles hitting the blades, which added complication and reduced the rate of fire. These remarks are of course concerned with conventional front-single-engined fighters with tractor propellers; even small twin-engined fighters could group several cannon in the nose (e.g. the Westland Whirlwind), and there were a few unconventional single-engined fighters with pusher props but these were not successful.

In the early 1930s Hispano-Suiza developed a liquid-cooled V12 engine with a 20 mm cannon firing through the propeller hub. It was almost essential to design or at least considerably adapt the engine specifically for such an installation, because the space between the cylinder banks (through which the gun barrel would need to run) would normally be filled with items such as intake ducting, carburettors or fuel injection systems, superchargers and other essential impedimenta. The propeller needed to be designed with a hole in the centre of the hub and its location also needed to be adjusted so that the hole lined up with the gun barrel. And of course, there was no hole through the centre of a radial engine, so such installations were impossible with this engine type. Evidently, it would not have been feasible to take an existing fighter designed for wing armament and fit it with an engine gun, without some major redesign.

Hispano-Suiza’s choice of moteur canon was the biggest of the Oerlikon cannon which it licence-manufactured under the HS 7 and HS 9 designations (the differences being minor). The engine and cannon combination was fitted into the Dewoitine D 501 fighter, which entered service in 1934, and subsequently into the D 510. The Oerlikon gun family continued to be developed even after this, with the guns becoming steadily lighter and faster-firing. By 1935 they were close to their ultimate in performance, in the FF, FFL and FFS, the last of these being a lightweight version of the Type SS AA gun. The smaller aircraft guns were primarily used by Germany as the MG-FF (the Oerlikon FF with minor modifications) and Japan, as the naval Type 99–1 (FF) and Type 99–2 (FFL). The earliest user, however, seems to have been Poland, which mounted two of the AF or FF cannon in the PZL P.24 from 1934 (service entry 1936).

Meanwhile, two inventors tried to improve on the Oerlikon API blowback design by introducing a locked breech which enabled the reciprocating bolt to be lighter and thereby faster-moving, to increase the rate of fire. The first of these was the Italian designer Alfredo Scotti, followed by the Swiss Marc Birkigt. Scotti developed more than one aircraft gun but his only success was the 20 mm AA cannon which made it into service. Birkigt, working at the French branch of Hispano Suiza, developed the 20 mm HS 404 aircraft gun whose test-bed performance was so impressive that it was not only adopted by France (it entered production in 1938, and in service with the M.S.406 and D 520) but also by the UK and the USA. However, it proved a troublesome weapon to develop, with various installation issues which took time to resolve, making it too late for the Battle of Britain. Like the Oerlikon guns, it was initially fed by a 60-round drum magazine, with belt feed entering service in RAF planes in 1941.

HS 404 mounted on engine block as designed

The Russian Polikarpov I-16 Tip 17 was another early user of contemporary cannon; this variant entered service in 1937 armed with a pair of 20 mm ShVAK in the wings; these enjoyed the benefit of belt feed, permitting a significant increase in ammunition load. Russia also acquired a licence to build the Hispano V12 engine, and became a prominent user of engine-mounted cannon.

Germany initially tried the French motor-cannon approach by fitting the very powerful 20 mm Rheinmetall-Borsig MG C/30L into a prototype He 112 fighter which was sent to Spain for combat testing. It was found to be effective against vehicles, but too slow-firing at 300–350 rpm for aerial combat, as well as too heavy. The ammunition feed was of interest: a 100-round drum mounted under the gun. The cannon were later put into use as light AA weapons, fitted with shorter barrels (the aircraft version had a very long barrel, to fill the space all the way to the propeller hub). While this aircraft installation was unsuccessful, the motor-cannon concept was later revived.

Rheinmetall-Borsig developed other aircraft guns at this time. A lot of effort went into evolving 20 mm cannon using less powerful ammunition than the MG C/30 L, resulting in the MG 204 of which only a small number was built and deployed by the Küstenfliegergruppen (coastal air service). More successful was the company’s 30 mm MK 101, firing very powerful ammunition. This was primarily used in ground attack, first being fitted to a dozen Messerschmitt Bf 110C-6 in 1940. Both guns were magazine-fed.

Other aircraft cannon developed in this period included:

•Madsen 23 mm: identical to the 20 mm except for the increased calibre: belt feed standard. Tested by various countries (including the USA) but only a small number were sold in South America.

•20 mm Furrer FMK 38: aircraft version of the AA gun described above, saw Swiss service both in engine-mounted installations and in twin-engined planes.

•37 mm M4, developed from a Browning design, which went on to see service in the Bell P-39 Airacobra.

Various French aircraft cannon were developed in the 1930s in 33 mm, 25 mm and 23 mm calibres, but apart from these being installed in a handful of aircraft mainly for test purposes, they saw little or no use.

Last but far from least, the MG 151/20 entered production in 1940 and went on to be the Luftwaffe’s principal aircraft cannon in World War II.

1941–1950

This section encompasses the period of the most urgent development of cannon at the height of their importance. Their principal use was against aircraft, for which purpose they were fitted to warships, ground vehicles, towed mountings, and other aircraft. Some use was also made of cannon for arming AFVs for ground fighting (although this developed to a far greater extent later), and for arming aircraft for ground or naval attack.

The primary concern was initially to obtain enough cannon which were suited to these purposes as well as meeting reliability requirements (particularly for installing in aircraft, when repairs and maintenance in flight were not feasible). Even with competent and experienced cannon engineers (a rare species) the process of designing, developing, debugging and manufacturing a new design took years. The cannon which had most impact in World War II were therefore those which were either already in production at the start of the war, or for which a great deal of development work had already taken place. Development during the war was mainly focused on incremental improvements to existing designs or, in some cases, a concentration on simplifying the manufacturing process to aid mass production.

In the years after World War II some radical innovations such as revolver and rotary cannon were developed, but with the loss of wartime urgency (and reduced resources) these were not introduced into service until the 1950s.

GROUND AND NAVAL ANTI-AIRCRAFT SYSTEMS

With a few exceptions, the cannon used in the AA role were multi-purpose, being used in land or naval mountings as required. Most of them had already emerged before World War II and were mentioned in the previous section.

Wartime experience revealed the practical superiority of the 20 mm Oerlikon and 40 mm Bofors over their contemporaries with their principal development emphasis being on simplifying manufacture for mass production (both guns initially required a lot of hand-finishing). Both the UK and USA were involved in this process; production time required for the Bofors dropped by around 50% while that for the Oerlikon was reduced by no less than 80%. The USN formally decided to adopt the 20 mm Oerlikon to replace the .50 Browning on 7 August 1941, at the same time as they decided to adopt the 40 mm Bofors to replace the 1.1-inch system.

A variety of naval mountings for the Oerlikon gun were developed for both the RN and USN. Most were single, unpowered, free-swinging mountings, but there were also twin mountings, both unpowered and powered. Some of the British powered Mk V twins were converted to take one 2-pounder Pom Pom, others to take a 40 mm Bofors gun (a system designated ‘Boffin’) as a result of the advent of kamikaze attacks, as they stood a much better chance of destroying the aircraft. Both navies also developed powered quad mountings, the Mk 15 for the USN (which saw limited service) and the Mk XIV, using belt-fed Oerlikons, for the RN (which did not, as the RN decided that fitting a pair of Bofors guns made a better use of the mounting).

Bofors STAAG mounting (NavWeaps)

The Polsten, a drastically simplified version of the Oerlikon, was used by the British army rather than the navy (presumably to avoid any logistics muddle) and was fitted to a variety of wheeled and vehicle mountings, including double, triple and quadruple (Canada having developed and manufactured the quad). It was in service between March 1944 and the 1950s.

The naval Bofors mountings saw considerable development in UK and US service. The RN equipment ranged from army-pattern air-cooled guns on single naval mountings (which made up most of the installations) to twin mountings featuring water-cooled barrels and in some cases stabilisation and even an on-mount radar.

While the 20 mm Oerlikon remained in USN and RN service in vast numbers to the end of the war, the British Army showed less interest in the light cannon, preferring improved models of 40 mm Bofors. This view was probably influenced by operational research into the relative effectiveness of the 20 mm Oerlikon and 40 mm Bofors in dealing with low-flying coastal raiders. It was noted that these were not usually spotted until 1,500 yards (1,370 m) away, so the Bofors could not use its range advantage. Engagement times were only about 13 seconds (from first sighting the target to the last shot) and the Bofors only had time to fire seven or eight rounds to the Oerlikon’s fifty. Even so, in most circumstances the Bofors was more effective.

Calculations were made about the probability of success (immediate break-off, or eventual crash) against a Junkers 88. Using simple sights only, the Oerlikon’s chances were estimated at 3% against a crossing target, 12% against one approaching head-on; the Bofors’ 18% and 20% respectively. With more advanced sights, performance was much better. Using predictor control, the Bofors’ success rate rose to 35% and 39%. The addition of a Triple Gyro sight had an even more dramatic effect on the Oerlikon’s performance – to 17% and an astonishing 76% respectively.

In USN service, the Bofors accounted for 33% of all Japanese aircraft shot down by AA fire during World War II, compared with 28% for the 20 mm, 30% for 5 inch guns, and 3% for MGs. The Bofors was credited with 50% of the total between October 1944 and March 1945, showing how its importance increased as the war progressed. In terms of the number of rounds fired for every plane shot down, the Bofors averaged 1,713; the 1.1-inch 1,932; the Oerlikon 5,287; HMGs (.50-inch cal/12.7 mm) 11,143; and MGs (.30’ cal/7.62 mm) 28,127. At the other end of the scale, the 5’ guns firing time-fuzed ammunition managed 654 rounds per plane; when firing proximity (VT) fuzes this improved to 340 rounds.

While generally considered inferior to the Oerlikon in the AA role, at the end of the war the Hispano HS 404 was being proposed for such use by the USN, for example in ‘Assembly 334’ with quad guns as an anti-kamikaze weapon capable of 2,800 rpm, but this was too late to see action in World War II.

Although Japan had criticisms of the 25 mm Type 96, it was nonetheless rated by users as the most reliable Japanese AA weapon of World War II. Late in the war it saw some use on land, with a two-wheel carriage being designed for it. The maximum effective range was 2,000 m, with the system being most effective at under 1,000 m. It took an average of 1,500 rounds to shoot down an aircraft at a range of 2,000 m and an altitude of 1,000 m.