When a number of shots are fired through a rifle small portions of the outside of the bullet are frequendy removed during its passage down the bore and left adhering to the surface of the bore. At first only the smallest particles of the bullet strip, but these may become almost fused on to the inside of the barrel by the extreme heat generated, and are not easily removed. The uneven projections thus formed scrape the surface of each successive bullet and gradually increase in size until the barrel becomes badly fouled with what is termed "metallic fouling."
And since metallic fouling may easily change the nature of the engraving on a bullet it is important that the possibilities should be appreciated.
The tendency of a barrel to collect metallic fouling depends on three main factors: the smoothness or roughness of the bore; the material of which the outside of the bullet is composed; the temperature.
It is but natural that a badly pitted and rough barrel will collect metallic fouling more readily than one which is in perfect condition.
Similarly nickel possesses a far greater affinity for the steel of the barrel than lead, or other materials which are used for the envelopes of bullets.
And the higher the temperature the more readily will nickel be fused to the surface of the bore. But it should be realised that temperature can be produced in several ways: the weather; the rapidity of fire; the nature of the powder; the gas pressure; and the velocity.
The first two of these ways are obvious and so need no further explanation, but the effect of powder and pressure are not so generally known. In Chapter IV it was explained that the gases generated by different types of powder varied in their temperature, while temperature was also seen to be a corollary of pressure.
In revolvers and pistols the powder charges are very small and the pressures low. Further, the velocities are much lower than those of modern rifles. So, other things being equal, there will never be the same tendency for revolvers or pistols to suffer from metallic fouling.
And probably in the case of most revolvers this tendency is now still further reduced by the fact that they fire lead bullets, for lead bullets never "lead" up a barrel as nickel-jacketed bullets will "nickel" one up.
I had imagined that this fact was too well known to need emphasis, but I have since heard an expert state in court that after thirteen shots with lead bullets a barrel would be so "leaded up" as to change completely its characteristics.
Now I have again and again fired a whole box of ammunition (276 rounds) in an afternoon without cleaning my revolver until the end of the firing. And yet I have never found any lead fouling which could be noticed except at the extreme breech end of the barrel. The leading here always takes the form of a ring, or part of a ring, at the very entrance of the barrel and is due to the passage of the bullet from the chamber (in the cylinder) into the bore.
In order to check my own experience I wrote to my friends, the late Lieut.-Colonel A. Whitty, D.S.O., and the late Major H. G. Lynch-Staunton. Both these officers had been Captains of the Army Eight, and as such had trained and coached teams for the Inter-Services Revolver Match at Bisley year after year, and thus had experience of many revolvers which is unique. Further, the experience of both officers went back for forty years.
Colonel Whitty replied as follows—
"I cannot say that I have ever found any real trouble from 'leading* in the bore of the service revolver; nor, during my many opportunities of observing others firing the ultra-rapid practices which have been so fashionable during post-War years, have I ever heard complaints from them of such trouble.
"My *455 revolver, which I got in 1902, must have had quite 15,000 rounds through it, but I have never had the bore treated for removal of lead, other than what I could do myself with the ordinary bristle brush. I have just this moment had a look at the bore and cannot detect the slightest sign of lead."
And Major Lynch-Staunton wrote—
"I entirely agree with your experience of lead fouling in revolver barrels, i.e. little, if any. And when I was shooting 200 or more rounds a day I never worried about cleaning until the end of the day/*
In self-loading pistols and those revolvers which can fire nickel-jacketed bullets the tendency to metallic fouling is admittedly increased, but here again there is no risk of nickelling from ten or a dozen shots fired with intervals in between so as to avoid heating the barrel unduly.
The alleged danger of metallic fouling is that the firing of more than one or two shots to obtain test bullets for comparing with a "crime" bullet may so change the surface of the bore that the striations on the final test bullet will differ from those on the first. But this danger is imaginary, and there is no risk of changing the thumb-mark of a barrel by accumulating metallic fouling from firing six to ten shots through any revolver or pistol.
The effects of wear, however, are in a different category. But first of all it should be appreciated that wear can have two separate effects on the inside of a barrel. It can enlarge the diameter of the bore, and it can change the minute markings on the surface of the bore which cause the striations in the engraving on a fired bullet.
Let us first consider the possibility of enlargement of the bore.
The higher the velocity of the bullet the greater must be the wear caused by every round, and consequently if we use rifles as examples we can be safe in the knowledge that the effects of wear will be considerably greater than in any pistol or revolver, and so our conclusions will err on the side of exaggeration.
By far the best method of checking the gradual enlargement of the bore of a rifle through wear is by adopting the normal routine of the scientific target shot. A part of his equipment is a set of barrel gauges, each of which is a perfect cylinder of a certain definite diameter which is measured accurately to a definite fraction of a thousandth of an inch. These gauges increases in size, so that a complete set for a -303 rifle will run from 3030 to -3045 or even 3050, each gauge being slightly larger in diameter than the preceding one. My own set runs from -3030 to 3040 in *oooi (nominal) at a time, and then to «3050 in 0-00025 (nominal) at a time.
Before shooting any series the barrel is cleaned and gauged, that is the size of the largest gauge, or plug, which it will take is noted. After the end of every series the barrel is cleaned again, and again gauged. It is then found that in course of time the barrel will take a gauge the next size larger, and this increase in the bore diameter which has brought about the change is due to wear.
Incidentally it may be stated that the size of the bore can, in some rifles, have a pronounced effect on the accuracy; and before now I have changed an indifferent shooting barrel into a first-class barrel by polishing out the bore carefully with diamantine until it was enlarged from -303 to -30325.
Now I have on numerous occasions in experimental shoots fired series of 60 consecutive shots at 1,100 yards, and I have never found that the bore of any of my rifles was enlarged by one of these series sufficiently to take the next larger gauge. In fact, I would place 200 rounds as the absolute minimum number of rounds required to enlarge the bore of a «303 rifle by '0001 of an inch. This number could be more than doubled in the case of any ordinary self-loading pistol before a similar enlargement could be produced.
I have heard it suggested that the firing of a number of test rounds might so enlarge the bore that the bullets fired at the end of the series would no longer bottom the grooves and so present a different characteristic engraving to those fired at the beginning of the series. The only reply to this is that, in the first place, it is the land diameter of the bore which is chiefly affected by wear, and so the bullets would be more likely to bottom the grooves at the end of a long series than at the beginning; and secondly that the series would have to consist of at least a thousand rounds before any noticeable effect could be produced» But this and similar statements do help to emphasise the really frightening ignorance of some "expert" witnesses.
It can, therefore, be accepted that the major characteristics of the bore cannot be changed by the wear produced by firing any ordinary number of test rounds in a suspect weapon.
But wear can, and does, have a decided effect on the minute markings on the surface of the bore which cause the finer striations in the engraving of a fired bullet,
I had long realised the possibility of this effect, and so carried out a number of experiments with various weapons. My experience has been that in the case of a revolver firing lead bullets, fifty rounds is sufficient to change completely the finer striations in the engraving. In some revolvers which marked their bullets with one or two very deep striations these particular striations could still be identified, although they were no longer so pronounced. But the finer striations had changed, and it was no longer possible to match the bullets perfectly with those which had been fired before the series of fifty rounds had been fired.
In the case of self-loading pistols firing nickel-jacketed bullets the change seems to be brought about more rapidly, and a series of but twenty-five rounds proved sufficient to change completely the finer striations in the engraving produced by a 7-63 mm. Mauser self-loading pistol. This pistol admittedly has a higher velocity than most self-loaders, which was my reason for using it for a test; but I feel certain in my own mind that no barrel can be relied upon to produce identical engraving after twenty rounds have been fired through it. It was for this reason that I stated in the last chapter that if bullet evidence of identification was to be utilised, not more than ten rounds in all should be fired through the barrel; five by the prosecution and five by the defence.
It must not be imagined that this type of wear will gradually leave the barrel perfectly smooth inside. As one lot of markings are rubbed away, fresh markings take their place, and so on. I have in my possession a -455 revolver through which I have fired more than 20,000 rounds. The bore is still in perfect condition, and bullets fired from it are marked with fine striations. But I know well enough that those striations are very different from those which must have been present on bullets which were fired from it when it was new.
Finally rust and corrosion can, and do, have an even greater effect on the inside of the bore even than good, honest wear. The weapons usually owned by criminals have generally passed through the care, or lack of care, of a good many owners; and it is usual to find both the barrels and the breech faces pitted with rust and often marked by cuts or scrapes resulting from careless cleaning. Naturally all such marks will leave their individuality on the fired bullet or cartridge case, although the barrel will probably change more rapidly than the breech face.
two photographs of the same lock taken from exactly the same angle, but with lenses of different focal lengths
The photograph marked A was taken with a 16-inch lens, and that marked B with a 6-inch lens. In A a streak of light can be seen between the lower curve of the hammer and the top edge of the lock plate, while in B the hammer overlaps the lock plate. Again, in A the protrusion of the sear tail below the bottom edge of the lock plate appears to be far greater than it does in B (see arrows) These two photographs emphasise the possibility of distortion in photography, particularly when lenses of different focal lengths are employed
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