A couple co-workers and I went to shoot our brand new Bushmaster XM-15 20" HBAR'S a while back. Mine worked flawlessly, even with the 10 round Franken mag. I always give my new guns a thorough cleaning before shooting. #1 - to learn how to take it apart, and #2 - they are usually dirty and fouled from test firing. Whatever oil is left over is usually dried up and gummy.
My co-worker's XM-15 jammed on the first round. I went over to investigate. I asked him if he cleaned it, he said no. The round was stuck halfway into the chamber, and I had to use the forward bolt assist to bang it in the rest of the way. I performed a quick field strip and cleaning. I loaded the same mag with 5 rounds. Bang, bang, bang, bang....bang. No problems!
Cleaning and lubing was the key.