B5 Systems Sierra K Extension Kit is the proprietary mounting system for the B5 Systems Sierra K buttstock. Includes a reduced-length, mil-spec diameter 7075 aluminum 5-position receiver extension, a short heavy buffer (3.5 oz), extra power spring, castle nut, and quick disconnect end plate. Made of mil-spec materials and finishes. Made in the USA. Weighs 3.50 oz. Safety note Follow all manufacturer installation instructions. Verify proper function and secure mounting before use. Comply with local firearm laws and regulations.