This kit will properly determine the groove diameter of handguns and rifles alike. From .380 ACP to .358 Winchester, this barrel slugging kit is specifically designed to determine the best fitting bullet for any bore diameter between 0.354" and 0.360". Modern firearms typically have rifling 4/1000th deep (0.004"). The SAAMI tolerance for a typical .35 caliber groove diameter is +0.002/-0.000 while the tolerance for the associated bullet is +0.000/-0.003". With these tolerances, a combination of large groove diameter coupled with small bullet diameter could leave only 0.0015" of rifling grip or worse with a worn barrel. A hot load could overcome that grip, fail to impart a proper spin, and prove inaccurate limiting the hand loader to weaker loads to achieve the accuracy they seek. Loading a properly fitted bullet will not only enhance accuracy but will seal the bore allowing the most effective propellant burn resulting in a much cleaner shooting firearm and wider choice of loads. This kit is easy to use, comes with complete instructions and will successfully determine your groove diameter using existing tools on your reloading bench. It is also warranted for 30 days. If it doesn't meet your expectations in any way, return it for a refund of its purchase price. You will like this kit!