This is a standard carbine-length buffer spring, the kind that ships in most AR-15 carbines. It drops into a carbine receiver extension behind the buffer and is what cycles the action and returns the bolt to battery. LUTH-AR lists it for .223 and 5.56 NATO carbines using an M4-type receiver extension.
There is nothing fancy here, and that is the point. A buffer spring is a wear part. It loses tension or kinks over time, and when it does, the fix is a fresh mil-spec spring. This is an inexpensive replacement to keep on the bench or in a parts kit.
One thing to know: this is a standard spring, not a tuned or polished competition spring. A shooter chasing a smoother, quieter cycle or fine recoil tuning should look at a centerless-ground option like the JP JPS-OSC instead. For a reliable factory-style replacement at a low price, this does the job.
