The Pacific Tool and Gauge Barrel Twist Gauge is designed to accurately measure the rifling of your barrel. The gauge works with 2, 4, or 6 groove barrels and features graduated markings for determining the actual twist of your barrel. With the barrel secured in a vise, the tool is inserted into the muzzle with the indicating collar pointing up. Then the tool is pushed down the bore of the barrel until the indicating collar has made one full revolution. After one full revolution the collar is slid down to the muzzle and the twist rate marking on the rod can be read. This tool is a much more precise method than the improvised cleaning rod and tight patch technique.