The depths of grooves is, of course, readily determined from the data obtained by the procedure described above. In the case of barrels having an even number of grooves the groove depth will be one half the difference between the groove and bore diameters. For barrels having an odd number of grooves the depth of the grooves will' be the difference between the groove diameter (as defined above) and the bore diameter.
Table 6 illustrates the reproducibility that may be obtained by the procedures described above. Reproducibility such as shown in this table can be obtained only in case of barrels that are in good condition (i.e., free from corrosion or rust) and obviously there must be no damage to the bore at the muzzle where the bore diameter measurement is made. In other words it must not be „out of round" or „burred." This difficulty, of course, does not appear in the case of guns having an even number of grooves because the measurements need not be made right at the muzzle end. Experience in the author's laboratory, and elsewhere, has shown that, in general, it is better to take measurements of bore and groove diameter at a point about one inch from the end of the barrel. This avoids any possible error due to „burring" at the muzzle or to excessive wear at the muzzle. This procedure was followed in the case of all barrels having an even number of grooves. Fortunately most guns have an even number of grooves, though there are notable exceptions.
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