Note: For guns having an odd number of grooves the difference between the bore diameter and the „groove diameter" represents the depth of the groove. The term „groove diameter", as here used, represents the distance from the bottom of a groove to the top of the opposite land.
In Type III the angle remains constant for a time, then increases at a regular rate for about two-thirds of the barrel length, then increases at a diminishing rate.
In Type IV the angle increases linearly for about two-thirds of the barrel length, then at a diminishing rate to the muzzle.
In Types V and VI we have plots that may be and probably are the results of mechanical difficulties. When these guns were made, about a hundred years ago, the art of rifling had not reached its present state of perfection. Inasmuch as no apparent useful purpose could be served by such rifling, it is presumed that the result achieved was unintentional. The author's guess is that these barrels were intended to be rifled similarly to those of Type I. The accompanying tabulations show the results of the measurements made, and the type of plot is indicated. While Type I predominates, it is believed that Types II, III, and IV also were intentional. Unfortunately we have no records which show what any of these types of rifling accomplished, if anything.
It seems from a study of these plots that the American manufacturers must have had in mind some of the considerations and claims found in the fairly recent German work. However, it is probable that the principal concern was the prevention of slippage due to the sudden encounter of the bullet with the rifling rather than the prevention of wear on the lands. The impression is that the first four types are definitely different experimental attempts to improve the ballistic performance of the gun. Whether they were based on sound theory or were just cut-and-try attempts one cannot know unless some records of the experimentation are found. The author realizes that it is not safe to generalize too much from the data obtained on such a relatively small number of barrels as were measured. One should have a very large number to measure to enable him to draw conclusions with certainty, but unfortunately they were not available.
The use of gain rifling in hand guns in the United States was of very short duration. Whether it was given up because of its ineffectiveness or because of the difficulties encountered in producing good rifling of this sort is not known, but one seems justified in concluding that both of these were contributing factors.
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