The accuracy of flight of a fired bullet depends to a considerable degree on its velocity, rate of spin, shape, and the perfection of that shape. If anything happens to deform the bullet or to make it at all imperfect in shape its flight will not be true.
When a cartridge is fired in a revolver the bullet jumps forward and acquires a very considerable velocity before it encounters the rifling which, presenting a curved contour, produces a change in direction. When the bullet strikes the rifling it has to follow the curve of the lands of the rifling because it fits closely. But because of its high velocity at the moment of impact it resists changing its direction, and there is always a tendency for the bullet to slip before it settles down to follow the rifling.
The result is that lead bullets fired from a revolver almost invariably show „skid marks" due to this slippage. jacketed bullets usually show it, but naturally the slippage is not so marked. This slippage produces a surface that is not perfectly symmetrical, as it should be for perfect flight of the bullet.
In a rifle or in an automatic pistol, on the other hand, we have a different situation since in guns of these types the forward end of the bullet is practically in contact with the beginning of the rifling and engages the rifling before it has acquired much velocity and momentum. Consequently slippage is usually absent.
The purpose of gain rifling, in which there is little or no angle of twist of the rifling at the breech but rather a gradual increase in angle of twist toward the muzzle, is to help the bullet to follow the rifling without slippage, as there is no sudden change in its direction after it has acquired considerable velocity.
The theory seems to be sound, and gain rifling was used in some of the revolvers made around the middle of the last century. But it was given up after a few years, perhaps partly because it was not easy to do a good job of rifling of this kind. It seems more likely, however, that it was not found to be effective in such short barrels.
There seems to be a great deal of variation from gun to gun and little standardization, in the cases studied at least. Also the shapes of the plots of the data indicate that there often was not a uniform change in the rifling pitch from breech to muzzle, as one would naturally expect. Furthermore, it was quite evident during the measurements that the rifling was not as „perfect" as in contemporary guns which do not have the gain rifling.
At any rate, this method of rifling did not survive, and in making measurements of over 2500 guns from 23 countries no modern guns were found to have this type of rifling. Gain rifling, therefore, as far as handguns are concerned, is only of historic interest and is not very likely to be encountered in criminal investigations-although the possibility exists, since many early Colts and Remingtons have been converted to take rim fire or center fire cartridges. An informed criminal might take advantage of this situation.
As already mentioned, the idea of gain rifling has long been known, but little regarding it is to be found in the literature. It has been used principally in heavy ordnance where it has been shown to be of considerable advantage in reducing the wear on the rifling, thus increasing the life of the barrels. In the case of a rifle barrel, or any barrel of considerable length, having a constant rifling pitch, the maximum wear occurs at the breech end of the barrel-say the first third of its length.
In this zone the temperature is highest and the gas pressures are greatest. If the rifling pitch is constant, as is the usual case, the rotational forces against the driving edges of the lands will be greatest in this region of high pressures, and consequently the wear of the lands will be the greatest in this zone. Continual wear on the lands eventually will result in slippage of the bullet, and it will not acquire the desired velocity nor the rotational spin necessary to produce stable flight.
Another point to be considered is the fact that bullets used in automatics (and many bullets for rifles) are jacketed with copper or an alloy which, being harder than lead, does not have as great a tendency to slip and to strip. There will be less fouling, but the wear on the rifling will be greater because of the greater hardness of the bullet metal. It would therefore appear to be advantageous to have the bullet achieve the desired rate of spin gradually rather than abruptly.
By using gain rifling, where the pitch of the rifling is zero or near zero at the breech and gradually increases toward the muzzle, it was thought that slippage and wear on the rifling might be reduced. The bullet, entering the rifling with high momentum, would be well „engraved" by the lands before it was subjected to a gradually increasing rotational torque required to give it the desired spin as it leaves the barrel. Thus gain rifling should do two things: (1) reduce slippage and (2) reduce the wear on the lands thus prolonging the life of the barrel.
Extensive experiments have been made in Germany and in this country on various types of gain rifling in the case of the larger-bore machine gun barrels and others where driving bands are used.
While in such cases there may be a decided advantage in the use of gain or „progressive" rifling, it seems most unlikely that it can ever be shown to be of any advantage in the case of revolver barrels. The changing angle would produce a broadening of the grooves made by the lands on the bullet and would tend to lead to inaccuracy and gas leakage. The shortness of the barrel would also be a deterrent factor.
The results of investigations on Colt and Remington revolvers, here reported, seem to indicate that these manufacturers had some of the abovementioned ideas about the possible advantages of gain rifling and did some experimenting along different lines.
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