I bought a new one (bushmaster, 20") and out of the box, it jammed every other round. Feh!! Turns out the feed ramp of the barrel locking ring was dead-sharp. After careful, very careful, smoothing, the problem vanished, never to return (knock on some big old wood, here). Also, in my zeal for cleanliness, I removed the peened-in-place bolts on the gas port of the bolt carrier. Now, they come loose once in a while at the range, causing short recoil. Stupid, I know, but I just make a habit of checking the tightness every time I clean it and keep an allen wrench of the appropriate size in my range bag! If I hadn't done that, it would be trouble free (mags are another story entirely)... Good luck on your problem.
John