Practical Ballistics E-Book

Lewis Potter

• Herausgeber: Crowood
• Kategorie: Lebensstil
• Sprache: Englisch

Seitenzahl: 240

Veröffentlichungsjahr: 2014

Beschreibung

This fascinating book starts with the basics, going back to the days of muzzle loading before considering the various attempts made over the centuries to improve the bullet. It moves on to examine the development of the cartridge right up to the present day, and provides the reader with a clear understanding of the principles governing the bullet's flight. Topics covered include the factors that influence ballistic performance and accuracy; 'internal ballistics' (factors influencing performance whilst the bullet is still in the barrel) and 'external ballistics' (factors affecting the bullet's flight when it leaves the barrel). Simple formulae are included to enhance the understanding of the principles involved. The book also describes a number of interesting experiments concerning performance and accuracy and presents the results. It aims to assist those enthusiasts who make their own bullets by analysing faults that have an impact on accuracy and covers both rifled firearms and smooth-bore guns.Includes a glossary that incorporates rifle terminology and those terms specific to the shotgun, and is superbly illustrated with over 100 colour photographs.

Leseprobe

Practical Ballistics

AN INTRODUCTORY GUIDE FOR RIFLE AND SHOTGUN SHOOTERS

LEWIS POTTER

THE CROWOOD PRESS

First published in 2014 by

The Crowood Press Ltd

Ramsbury, Marlborough

Wiltshire SN8 2HR

www.crowood.com

This e-book first published in 2014

© Lewis Potter 2014

All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publishers.

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library.

ISBN: 978 1 84797 738 0

Disclaimer

The author and the publisher do not accept any responsibility in any manner whatsoever for any error or omission, or any loss, damage, injury, adverse outcome, or liability of any kind incurred as a result of the use of any of the information contained in this book, or reliance upon it. If in doubt about any aspect of ballistics readers are advised to seek professional advice.

All images are by the author except where otherwise stated.

Contents

Acknowledgements

Preface

1

The Bullet and Cartridge

2

Internal Ballistics and Associated Matters

3

External Ballistics and Associated Matters

4

Practical Matters Affecting Performance and Accuracy

5

The Smooth-Bore Gun

6

Smooth-Bore Ballistics

Glossary

Index

Acknowledgements

A book, especially with technical content, is rarely the work of one individual and most authors are only too glad of help, advice and assistance offered by others, from reviewing the manuscript to help with practical testing.

I therefore acknowledge the kind help and assistance from the staff of the Birmingham Proof House, the Greensleeves Shooting Club, Paul Edmunds, Paul Harding, Patrick and Paul Faulkner, Alan Cox, Geoff Paskin, Martin Taylor and my son, Dan Walker. Special thanks go to Chris Price, who, for the third time, volunteered to review a manuscript for me; also Derek Allsop and Dr Geoffrey Kolbe, who gave invaluable advice; I also appreciate the kind help given by Eley Hawk Ltd.

I am, as always, extremely grateful to my wife, Sue, who, for this fifth book of mine, has worked so hard converting my ever-increasingly scruffy handwriting into a legible typed manuscript.

I hope I have not failed to thank anyone, but if I have please accept my apologies as any such oversight will be down to my deficient memory,which I find does not improve with age.

Preface

The idea behind this book was to make the fairly complex world of ballistics easier to understand, and to relate in a practical manner to the kind of use the sportsman or target shooter has for their rifle and ammunition. It is nonetheless a subject that does require a degree of calculation, hence the fairly simple formulae included to enhance understanding of the principles involved.

If there is a single conclusion from these writings it is that there is no such thing as the perfect all-round cartridge, although some may come close for general target and live game use. However, when it comes to bullet design it is a very different matter, and we are drawn inevitably to accept that different designs can have widely different applications to perform at their best. So often it is a case of ‘horses for courses’.

I have included reference to black powder firearms and the associated large calibre heavy bullets, as they have their uses even in a world dominated by high velocities and pointed bullets. The older technology tends to be a neglected subject, which I feel is a shame as it is all of interest and the basis of what was to come later.

In the same manner it was most useful, I believed, to include ballistic matters concerning the smooth-bored shotgun. All too often rifled and smooth-bored firearms are treated at different subjects, although a lot of shooters use both. I trust I have made the world of ballistics a little easier to understand and especially the practical application. That, however, is for you, the reader, to judge.

LASP 2013

Chapter 1

The Bullet and Cartridge

Pure ballistics tends to be thought of as a dry subject typified by complex formulae, the sort of specialization once described as knowing more and more about less and less. This is a little unkind perhaps, and some understanding of basic calculations is essential to have a reasonable comprehension of the subject, but what many shooters want to know is on a very practical level, such as ‘if I alter A, how does it affect B?’ – quite simply, what factors affect performance and accuracy. These can be many, apart from the obvious, such as rifling type and twist rate, bullet form and weight, right through the gamut of problems including head space, deformed cartridge cases and misshapen bullets.

Therefore this book is mainly about the practical aspects of ballistics: what affects performance and how to improve that, while hopefully avoiding some of the pitfalls. To get a better understanding of this it is first useful to know something of the development of the bullet and cartridge.

BASIC MUZZLE-LOADING FORMS

The bullet was at first a simple lead ball, its accuracy of manufacture limited by the precision or otherwise of the mould and the skill of the maker. Having examined old ball moulds one comes to the conclusion that it was, for many years, an imperfectly formed but nonetheless practical projectile, simply the best there was in the early days of firearms development. Just when the ball originated is difficult to establish, but it has probably been in use for around seven centuries as it is still used today.

In its early use in smooth-bore guns the ball would sit either directly against the propellant (black) powder, or between an over-powder wad or wadding with a further wad on top of the ball to prevent it rolling out of the barrel if the gun were held with the muzzle at a depressed angle. An alternative to this would have been to wrap the ball in a cloth patch to make it a snug fit in the bore, which, as we shall see in a later chapter, can have a remarkably detrimental effect on accuracy.

While originally the propellant gun powder (black powder) was held in bulk in a separate container typically made from wood or horn and later copper or brass, one quick loading development (a relative term) was individual powder charges in stoppered containers. This effectively was the start of the evolution of the cartridge, and was followed by the first almost self-contained cartridge with the ball at one end of a sealed paper tube and the powder behind it. The British used this system for the famous .75in-calibre flintlock musket affectionately known as Brown Bess.

The procedure was to bite or tear off paper from the cartridge and pour a small quantity of powder into the pan, close it, then pour the rest down the barrel followed by ramming home the ball in its paper tube that then acted as wadding. Reputedly some four shots a minute could be achieved with much practice where volume of fire, not accuracy, was the most desirable attribute.

The problem with a rifled muzzle-loading firearm is that ideally the ball needs to be sized to fully engage the rifling at the breech end of the barrel. This means hard work loading, including the use of a mallet on the ramrod or a short starter rod to get the ball to engage the rifling at the muzzle. Then there are other complications, such as most muzzle-loading barrels would have been finished prior to the addition of the rifling by a process known as spill boring. Spill boring produces a tapered bore, the amount dependent upon the number of passes of the tool, how much material is removed, and the length of the barrel, but this taper can vary from a few tenths of thousandths of an inch to two to three thousandths.

Scissor ball mould. The cutter for removing the casting sprue that gives this type of mould its name is just at the rear of the pivot pin.

It was believed, and appears to be the case with low velocity arms, that the slightly tapered or choked bore was beneficial to accuracy, the only complication here being that the breech end is, of course, then going to be larger than at the muzzle. This means the ball will not be such a precise fit, even if engaged nicely in the rifling at the muzzle, and the force required to start the ball can make it out of round and therefore ballistically inefficient.

Speed of loading and reloading, especially for military purposes, is always an issue, and one crude method occasionally resorted to was to use a calibre-sized ball (‘calibre’ being the bore size prior to the addition of the rifling) that would slide down the barrel without engaging the rifling. Once seated against the powder, a hard blow or two with a steel ramrod would partially expand the ball sideways into the rifling. This had the twin disadvantages of compressing the gun powder into a virtually solid mass, which is not an aid to good, consistent ignition, and once again producing a deformed ball that will not fly as true as a spherical one.

The answer, as so often is the case, was stunningly simple – the patched ball. This works well in a rifled firearm with the slightly undersized ball, the lubricated patch gripping the rifling while protecting the ball. On firing, the patch falls away as it leaves the muzzle while the unmarked ball spins its way to the target. With a comparatively light ball, as compared to a conical bullet, the rifling twist can be fairly slow, which is another aid to loading as it causes less resistance than a faster rate of twist. In many ways, crude as this technology may seem when viewed from the perspective of the twenty-first century, it was the origin of the jacketed bullet.

For years powder was carried in a separate container. This is a fairly late Hawkesley-made leather-over-copper powder flask, the adjustable nozzle graduated in grains for rifle use.

One of the advantages of rifling is its ability to handle conical-nosed bullets with a long cylindrical body and therefore of greater weight than a ball of the same calibre. This has many advantages, including a more ballistically efficient shape, greater muzzle energy, more retained energy at longer ranges, and better terminal performance with its superior sectional density (ratio of diameter to length). The problems of loading involving the rifled muzzle loader are much the same as when using a ball, but somewhat more exaggerated.

For the target shooter, where time was of far less consequence, one answer was the paper patched bullet. This was achieved by attaching to the muzzle while loading, a smooth-bore ‘false muzzle’ cut to accommodate strips of overlapping paper; the bullet was placed over the paper, and was then pushed or rammed into the bore. Like this the paper forms a jacket around the bullet, and in the same way as the patched ball, grips the rifling, then falls away, rather like a sabot, when the bullet leaves the muzzle.

The disadvantage with this system was the time needed for careful loading, and on a windy day it must have been quite challenging keeping all those strips of paper in place before entering the bullet into the false muzzle. As was so often the case for centuries, the prime mover in firearms development is advancement in efficiency in weapons of war, meaning firepower, so something less fussy was required.

Ball and lubricated patch, the first step towards the jacketed bullet.

THE MECHANICALLY FITTED BULLET

There are two well known candidates in this category: Colonel John Jacob’s winged bullet (not the first of its type, but the final and most successful development), and Sir Joseph Whitworth’s hexagonal bullet. The former has projecting lugs that engage the rifling so the body of the bullet can be calibre size, which again is a great aid to loading. Once again, however, from our perspective it may seem odd that someone should design a bullet with effectively short, stubby paddles sticking out on either side, inevitably creating turbulence as the bullet spins through the air.

To a certain extent it was a case of ‘needs must’, where a degree of accuracy was offset against efficiency of loading. Also, at subsonic velocities and with comparatively slow rates of rifling twist, slight imperfections in bullet design or construction tend to have a lesser effect than with bullets driven at supersonic speeds. Thus Jacob’s bullet seems to have found its niche in larger calibre rifles such as those used for dangerous game usually shot at short range.

Whitworth’s hexagonal bullet looks like a blunt-nosed, slightly twisted piece of hexagon bar which is formed to mate with the hexagonal rifling. An idea also applicable to artillery shells, in small arms it became a favourite once again of the long-range target shooters. Such was the fame of the Whitworth rifle that the Birmingham firm of Parker-Hale reintroduced it as one of their range of modern muzzle-loading rifles. However, while their Enfield rifles were manufactured to the original gauges, the Whitworth ‘.451 Volunteer’ was not quite the same as an original, even if a worthy effort.

Mechanically fitted bullets. Jacob’s winged bullet was originally designed with four lugs, later two, that engaged into deep-cut rifling. Whitworth’s hexagonal bullet fitted rifling of the same form.

The Minié Bullet

The great breakthrough which gave speed of loading and acceptable accuracy was the invention of what is popularly known as the minié bullet, another of those ideas of brilliant simplicity that crop up from time to time. The principle of a comparatively loose-fitting bullet that would expand to grip the rifling as a result of the pressure applied by the burning propellant was first experimented with as early as 1823, and over the next thirty years or so there were several variations in design culminating in a simple hollow base design that was actually an improvement on the minié of 1849.

The great advantage of this type of bullet in a muzzle loader is that it can be loaded as quickly as a smooth-bore musket but has the accuracy inherent in a rifled firearm. Almost at a stroke the smoothbore musket became obsolete, and the next need was for a practical breech-loading system.

The minié bullet and its variants revolutionized loading a rifle. The bullet has a large hollow base which at different times was fitted with an iron cup or wooden plug to aid expansion into the rifling.

LOADING AT THE BREECH

There were two significant steps forward concerning rifle accuracy and a greater appreciation of ballistics. The first was loading at the breech, and the second was replacing the black powder with smokeless propellant, which produced considerably less fouling of the bore. Even though by the late nineteenth century black powder was the very best of its type, and is hardly matched even today, the deposits after firing have an adverse effect on accuracy and loading. While as a general rule this also varies with the quality of the powder, on a hot day, or when the barrel heats up with use, the burnt residue becomes crusty and hard, though less so on a cool, damp day. Nevertheless in all situations it means loading becomes progressively harder, especially with a muzzle loader, and it even affects breech loaders when it builds up in the lead into the rifling.

I once found problems when using a black powder breech loader in competition in a rapid fire stage with powder that was not of the best quality. After a few shots the occasional ‘flyer’ was recorded on the target, well outside the rest of the group. After twelve shots one of these flyers became so unstable in flight it emitted that distinctive buzzing sound as it tumbled down range. It seemed in this instance that the powder fouling had become sufficiently hard to tear the paper patching so badly that the bullet left the muzzle with insufficient spin to stabilize it. This seemed to be borne out by shooting the gun later and cleaning between shots, when good accuracy was restored.

A variety of bullet designs. From left: ball; cast lead with gas check; full metal jacket (FMJ) cutaway to show the turn-over at the base; cutaway target hollow point – the jacket covering the base.

Part of the problem could also be attributed to lack of bullet lubricant, because cast bullets with large lubrication grooves, rather than the grease wad (what the Americans call a grease ‘cookie’) behind a paper-patched bullet, performed much better with this particular black powder. Still, it was an interesting lesson in how other matters can affect quite basic ballistics, even with good bullets and excellent rifling. For centuries, though, black powder as a propellant held back the development of smaller bore rifles and the kind of higher muzzle velocities that could, finally, be obtained with smokeless powders.

With smokeless powder and jacketed bullets the world of shooting changed once again. Where previously .450in had been a medium calibre and .360in regarded as small, sizes around .30in calibre became the norm. Almost overnight velocities were up by around 50 per cent over the very best performing black powder loads, and nearly double many of the older loadings.

Loading at the breech also contributed greatly towards accuracy and more consistent ballistics. Although there had been experiments with breech loaders as early as the sixteenth century, it was never a truly practical proposition until the advent of the metallic cartridge. The capping breechloader led the way with its paper cartridge loaded at the breech, although it was still ignited by a percussion cap on a nipple mounted on the breech block. An early metallic cartridge rifle like the 1874 Sharps was a simple redevelopment of the earlier capping breech loader, and, of course, was still fuelled by black powder.

The gyrojet: was this to be the future, essentially a rocket-propelled bullet? It was an interesting concept that did not catch on. It is shown here with a.303 British full metal jacket military bullet for size comparison.

However, put smokeless powder and the metallic cartridge together, and many things are possible, such as carefully designed jacketed bullets; and because there is such a lack of powder fouling, each shot is virtually consistently the same as the preceding, except sometimes when firing from a cold, clean barrel, or when it is overheated.

Higher velocities meant the adoption of jacketed bullets, at first called copper patched, following on from paper patched. The need, of course, for jacketing was that without it even hard lead alloy bullets used at higher velocities would strip the rifling, depositing lead in the bore and having no spin to impart stability. What might be described as a ‘mini-jacket’ is the gas check which fits over the base of a lead bullet to protect it from the hot, high pressure gases, especially those generated when using smokeless powders. Using a jacket means that the bullet is less prone to damage during handling; it also means that bullets with very precise manufacturing tolerances and a fine streamlined shape can perform better, especially at the longer ranges. The metallic cartridge as we know it had arrived.

That is not quite the end of the story, as inventors push ideas even further, such as caseless rounds, or that slightly crazy idea, the gyrojet, a sort of mini-rocket self-propelled bullet – perhaps rather a triumph of ingenuity over practicality.

THE METALLIC CARTRIDGE

For almost all of us, the metallic cartridge is the ultimate development of the cartridge in all its various modern and some not so modern forms. The basic types likely to be encountered are the recessed rim (more commonly called ‘rimless’); the rebated rim, where the recessed rim is smaller than the main diameter of the body of the case; the belted, which is effectively a forward- located rim on a recessed rim case, which also provides useful reinforcing; and that oldie, the rimmed case. Although of all of these the rimmed case has been around the longest, it is by no means technically inferior when used for specialist applications such as break-open or falling block rifles. It is not well suited to magazine rifles, which are much in the majority-hence the great popularity of the recessed rim cartridge.

With these cartridge cases two types of primer are in use, the most common and useful being the Boxer primer named after Colonel Edward Mounier Boxer, a British officer; this is a self-contained design sitting in the primer pocket. The American Hiram Berdan’s system consists of a hollow cup with the anvil formed in the base of the primer pocket, and two flash holes either side. The Berdan is little used except mainly for military applications, and is of so little use to the keen hand loader that it can be discounted for practical purposes – although forty years ago we sometimes had to use it due to a lack of the older types of cartridge case.

In recent years the supply of cases for obsolete cartridges has improved enormously thanks to specialist makers, all of these modern supplies being Boxer primed regardless of how the originals were produced. Even the rimfire has had something of a revival. The .22 Winchester WMR, commonly referred to as the .22 magnum, was introduced in 1959, with the recent development of the rimfire cartridges being the .17 HMR.

The three common rim types: from left: rimless (recessed rim), rimmed and belted..

TERMINOLOGY

To understand the basic principles concerning firearms and ballistics it is very useful, even essential, to have an appreciation of the correct terminology. The language relating to firearms is sometimes a little archaic. It has slowly evolved over the years very much at its own pace, and sometimes almost independent to mainstream English. One typical example is the British gunmaker or gunsmith referring to a turnscrew – what most people in the English-speaking world would describe as a screwdriver. The word ‘turnscrew’ was common in Victorian times, and while everyone else adopted the later description, the gun trade doggedly stuck with what they were familiar with, about a century out of step with the modern world but nonetheless demonstrating a charmingly old-fashioned way of dealing with progress.

Such old language is more common in the shotgun world, but even relating to the rifled firearm there can be confusion as to the correct descriptions and terminology. Partly to blame are internet chat sites and forums where it seems that participants, when they do not fully understand the subject, tend to adopt words that sound about right, or simply make things up. This is flashed around the world in seconds, and it is quite possible for information that might be untrue to be given credibility simply because it appears on the internet, which in itself is a wonderful tool when used correctly.

Therefore, for the purposes of this book, ‘cartridge’ is used as a generic term for both the metallic rifle cartridge and the paper- or plasticcased shotgun cartridge. ‘Bullet’ is used for any elongated projectile, ‘ball’ is reserved for spherical bullets simply to distinguish between the two, while ‘shot’ describes multiple small pellets or balls as used in a shotgun.

Calibre and Gauge

To be strictly correct, ‘calibre’ is the bore of a rifle prior to the addition of rifling, with the exception of ‘gauge’ rifles, which are based on the old shotgun system of measurement. The use of the word ‘calibre’ has been extended in common usage over the years to describe not just the calibre, but also the cartridge a rifle is chambered for, and this has gone on to cause confusion so perhaps requires a bit more explanation. An example of this would be ‘rifle calibre .270 Winchester’; however, to use it the other way around, as in ‘.270 Winchester calibre rifle’, the use of calibre is then redundant, little more, in fact, than an affectation, as ‘.270 Winchester rifle’ gives you all the information necessary.

Where it really starts to go wrong is when ‘calibre’ is used as a direct substitute for ‘cartridges’. It is not unusual to come across a description like, say, the 30-06 Springfield cartridge described as ‘a good old calibre dating from 1906’. It is, of course, a fine old cartridge, and the use of ‘calibre’ in this instance is quite wrong. There are plenty of others in .30 calibre, some predating it, some of later development, some not so good.

Those big old rifles, both muzzle and breech loaders, that may be referred to as ‘gauge rifles’ are based on the old spherical ball system of measurement common to shotguns and, at one time, cannons. With small arms the size referred to the number of spherical lead balls of bore diameter that go to make up one pound in weight. So it is easy to see that a spherical ball that is 1/16lb means ‘16 gauge’. A large shotgun or rifle that accepts a lead ball weighing 1/4lb, or four to the pound, is gauged at 4. Whether to use gauge or bore is another matter. The older system, which the Americans still tend to use, is gauge, while bore is most common in the UK – yet a gunsmith will still refer to ‘gauging the bore’.

To use ‘calibre’ in this instance is not logical as it denotes a direct measurement, while what we actually have is a comparative measurement relating size to weight. Even that great showman of the British gun trade, W. W. Greener, was inclined to use terms such as ‘calibre 12 bore’, but in this instance I believe he was not actually at his technical best. One exception, of course, is the .410, which refers to a direct measurement or calibre.

For the sake of clarity I have used ‘bore’ to denote the size of gauge rifles and shotguns because, while the latter is probably the more technically proper, ‘bore’ is an acceptable substitute and is well established in the UK. ‘Calibre’ I have continued to use as meaning a bore measurement, rather than describing the cartridge for which a rifle is chambered.

Lead, Throat or Leed

The lead of the rifling is the tapered section in front of the chambered bullet that literally leads it into the full depth of rifling. There has, in the past, been some debate as to whether it should be spelt ‘leed’, which has tended to be the case with shotguns – and when a sentence or paragraph also includes lead, meaning the dense, soft metal that is the basis of most bullets, the usefulness of an alternative spelling becomes fairly obvious. However, ‘lead’, regardless of the possibility of confusion, of which there are plenty of examples in the English language, appears to be the established and recognized spelling when referring to this part of a rifle barrel.

So, is the lead the same as the throat? Some authorities would say yes: after all, when we wish to alter the lead we use a throating reamer; but it is another possible area of mild confusion if the two terms are used interchangeably. For the purposes of this book I have stayed with ‘lead’ as meaning the whole tapered section, and ‘throat’ just being the start of the rifling where erosion – or throat erosion – first becomes really visible.

Backstop/Backdrop

Reference will occasionally appear in the text to ‘backstop’, meaning a natural or artificial barrier used to arrest the bullet’s flight within a certain distance, and referred to on a rifle range as a ‘butt stop’. It is to be noted that ‘backdrop’ is being used increasingly on the internet and sporting press: however, the origin of ‘backdrop’ is the curtain at the rear of a theatre stage which would often carry a scenic picture. It may also be correctly used to describe the actual scenery behind something that is the focal point of a picture, as in ‘he was pictured with the Malvern Hills as a backdrop’. It is not what a backstop is to a rifle bullet, and I have included this explanation to avoid confusion, as the two terms are actually not interchangeable.

Rifle Action

Another area of slight confusion, before we get down to the real detail, relates to parts of a rifle. The word ‘action’, often used as a suffix as in ‘bolt action’ (but sometimes used on its own) is really meant to describe the complete working assembly of parts – bolt, receiver, firing pin, springs, cocking piece – and how it functions. One sometimes hears the receiver being described as the action, but it can be seen from the foregoing that this is not actually correct: it is only one part, although a major pressure-containing part, along with the bolt and barrel.