I met a friend at the range yesterday so he could test fire his new AR. Some rounds would not fire, no indentations on the primer, or it would fail to feed. So, I had him try a different mag to eliminate that as the problem. It ran fine for a few rounds then started jamming again. Then, a case got stuck in the chamber, and the bolt would not move. So, I separated the upper from the lower. When I did, the buffer and buffer spring popped out. When I removed them, I saw that the buffer retainer was missing, and the retainer spring was laying in the trigger group. He had installed a sling adapter and when he replaced the buffer tube, he had overlooked it. When he got home, he found the retainer under a table. It goes to show, that you need to pay attention when working on your firearms.