In the course of this study of rifling characteristics, in which over 2500 handguns (revolvers, pistols, and automatics) have been measured, a number of revolvers have been found that have „gain" rifling, i.e., the rate of turn increases from breech to muzzle. Although such rifling has been known for a long time, it does not appear to have been used in handguns except for a few models of fairly early Colts and Remingtons. Its use was soon given up, but there are in existence today, in museums and in private collections, many guns having this type of rifling.
The guns that have gain rifling are of the percussion type, though some of them have been converted to rim fire or center fire. Percussion-type guns are not likely to be used by the criminal today because it is so much easier to use cartridges. However, these older guns do have interest for the collector or specialist.
The guns were, for the most part, obtained from the Rosebush Collection in the museum of the Wisconsin State Historical Society at Madison, Wis. A few came from the Nunnemacher Collection in the Milwaukee Public Museum and some from private collectors. Many of the guns examined had rifling which was too corroded to allow reliable measurements to be made and consequently were not measured.
The twist of the rifling was measured by means of the „Rifling Meter"-an instrument designed and built at the University of Wisconsin and described in the chapter on Instrumentation.
In using this instrument, a lead disk about 1/16 inch thick and of suitable size (slightly larger than the bore) is forced through the barrel to be measured and readings of angular rotation of the disk (which follows the rifling) are made at equally spaced intervals-the length of interval depending on the length of the barrel and upon whether the barrel has gain rifling or not. The disk is mounted on the end of a „thrust rod" which in turn is held in the center (or hub) of a graduated circle which is mounted on ball bearings so that friction is reduced practically to zero.
In the case of gain rifling it has been found desirable to make measurements at short intervals, usually 0.2 inch, and to plot the successive readings of rotation against the length of barrel traversed. Measurements can be made from either end (if the barrel can be removed), and wherever possible they were made from both ends and the readings were plotted on the same graph. If the two plots coincide where they overlap it serves as a good check on the accuracy of the measurements.
Inasmuch as the rifling is more likely to have defects near the ends of the barrel, the readings near the end may be in slight error, and an extrapolation of the curve will give more accurate results for values in these regions. In a number of cases it was found desirable to use the extrapolated value-as indicated in the accompanying plots of data. In a few cases the condition of the barrel, at one end or the other, necessitated more extrapolation than is desirable, but the error certainly is not great. In the case of guns having barrels longer than the thrust rod, measurements were made from each end, wherever possible, and the two plots combined in case they overlapped in the center section. The methods used to measure bore diameters, groove diameters, and land widths are described elsewhere in this book. (See chapter on Instrumentation.)
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