Ruger AR-15 M4 Stock Kit

Ruger sells this as a parts kit for AR-15 pattern rifles: an M4 style collapsible buttstock, a carbine buffer, the recoil spring, a castle nut and an end plate. Those are the pieces that normally get replaced together during a stock swap, so it saves picking through a bin of small parts to find them one at a time.

The M4 stock adjusts along a carbine receiver extension, so length of pull moves with a plate carrier or a heavy coat. Ruger notes that AR-15 components vary widely between makers and that fit cannot be guaranteed across every rifle on the market, so a gunsmith is the safe route if you have not done a stock swap before.