To hear akaindy tell it, he removed the magazine and attempted to clear the jam by racking the bolt, then by reaching up through the magwell and trying to manually dislodge the cartridges, to no avail. The gun was then taken off the line, the dustcover, recoil spring and bolt group were removed, and the cartridges were forcibly extracted from the chamber mouth.
So what was the mag type he was using? I agree with Snorko and Indykid, my bet would be on the mag.