Picked up a S. Korean 75 round drum and when attempting to chamber, the bolt gets stuck on the top of the round. Won't go any further. Played with the spring tension on the drum, etc. and it doesn't matter. With zero tension the bolt will go all the way back (of course no round is being chambered). I've tried it both on my Norinco and my M10, and it sticks in exactly the same spot on both of them. It DOES seat perfectly into place on both rifles. I have several different flavors of mags that work perfectly, so this is unique to the drum. Any thoughts on how this might be remedied? Appreciate any input.