You can have rim-lock with any of the rounds. The top round rim must be in front of the lower round to feed. Also, after I fully disassembled one of my MN's to wash out the cosmolineicon, when I reassembled the rifle, I found there was a bit of slide room with the Interrupter/ejector that might have been the cause for a similar issue. I just unscrewed the screw a little, used the screwdriver to drift the I/E further to the rear just a touch and it worked fine afterwards. I don't know if it was the issue to cause the lock (no overlapping rim-lock found), but the problem went away. Good luck.