Ruger AR-15 Buffer Tube Kit

Everything behind the lower receiver in one box. The kit is the carbine receiver extension plus the hardware that goes with it: buffer, recoil spring, castle nut and end plate. That is the full assembly for a collapsible stock build, so you are not chasing down a castle nut or a spring separately after the tube shows up.

Ruger makes these for AR-15 pattern rifles. Component fit across the AR market varies more than people expect, and Ruger does not guarantee compatibility with every lower and stock combination out there, so check yours before you stake the nut. If you have never assembled a receiver extension, a gunsmith will have it done in minutes.