These barrel slugging kits are specifically designed to determine the proper bullet diameter for a .38-55 caliber firearm. They will also determine the best fitting bullet for any bore diameter between 0.375" and 0.380". Since 1876 the .38-55's groove diameter varied widely and accuracy depends on the proper size bullet to seal the bore and grip the rifling to impart a stabilizing spin. Good bores with groove diameters as tight as 0.375” and as loose as 0.380”can be found. Worn rifling needs the bullet to completely fill the groove diameter to have any hope of ample grip. Loading the proper bullet will not only enhance accuracy but will seal the bore allowing the most effective propellant burn resulting in a much cleaner shooting firearm. 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!