Nobody makes bases to fit as many guns as Weaver. Yet each base is machined to tight tolerances for a custom fit. They"re designed to fight the effects of recoil with a square-cut notch that precisely mates to the sturdy cross bolt in every Weaver Detachable Ring. Custom-hardened fastening screws come with every Weaver base.One piece base used on:- Thompson Center: Contender Handgun, Carbine- Rossi/Braztech: .22LR/.410 and .17HMR/.410 Small FrameMatched pair combo onlyNote: Base does not fit Rossi large frame matched pair combo firearms (all models with 20 ga. or 12 ga. shotgun barrels)