Re: Buffer retainer
SWPC, you got two fine answers! Not being that familiar, as you said, with the ARs, the first thing you have to do is remove the butt stock. If it's the older A-2 , solid model, there's a big screw on the top of the butt to take out. Then the stock will slide off. There's a "plug" inside, so don't loose or misoreintate it if it falls out. Can't go but one way anyways. Once you have the stock off, you'll see a built in nut on the recoil tube. Now, look at the pin at the other end and see how it's in relationship to the buffer. It should be only the pin against it. The tube should be far enough turned in to go over the base of the pin and only very close to the pin, practically touching it. That allows you to push the pin down which will release the buffer and spring. That's also the way you align the tube when re-installing it. You have to push down on the pin so you can tighten the tube up to the pin and over it's base.
Now, the collapsible stocks are slightly different. You can still see the relationship of the pin to the buffer. These stocks have a nut and retainer on them. Loosen the retainer nut to un-screw the stock. Needs a spanner wrench.
One answer you got was to check the pin and it's spring. Yep, the pins sometimes break off. Sometimes the tube isn't in far enough to hold the pin down and as the bolt moves rearward it passes the pin and it will dislodge from it's hole. Either way , it's an easy fix.