The rifle: BCM mid-length upper w/ 16" barrel, full-auto BCG (BCM), Carbine Buffer (3 oz), unknown buffer spring composition. I don't have a solid round-count on the upper, but it's likely in the 1000-2000 round range. Up until the problem it's experiencing now, it has run flawlessly with XM193 and M855. Condition: Cleaned and inspected after every range trip, breakfree used to clean/lube, along with a light application of remoil on the portions of the bolt assembly. No lube or cleaning of the gas tube. The magazines: 20-round brownells aluminum mags with stock followers, and 30-round brownells aluminum mags with magpul followers. The ammo: American Eagle XM-193, approximately 8 years old. The problem: After approximately 60 good rounds down the pipe this weekend, it began intermittently (randomly) wedging a round between the bolt and the chamber. I'm including a picture of the problem I found on the internet, which depicts my problem, but note the pic is NOT my rifle (no way I'd grease the bolt like that). There was no problem ejecting the round before each jam, though I did not have the opportunity to view ejection to see if the spent casings were being thrown with the same force/direction as when a jam did not occur. Upon clearing the jam, most of the jammed shells had a pretty good dent in the case from where the bolt contacted the case body. I know this problem occurred with the 20-round mags. Unfortunately, I don't know for sure that it did with the 30-round mags, though if memory serves it occurred with both. So, Highroad Oracle... whatcha got for me? I have a couple ideas of what might be happening, but I need some help with this one.