Diameter of center hole is fits .22 to .45 . Designed to be used with our Hrbrid Trap kit, this fixtureallows for easy drilling cups and end cap of our traps. 22 caliber - 9/32 - 0.2813" 30 caliber - 3/8 - 0.375" Utilizing a hardened steel drill bushing this fixture will give youyears of use..syi