Thanks for the response.

Yes, I've tried loading only 1 round in the magazine, and it failed. Don't remember whether it was on a closed or open bolt, as I tried it a few times.

The bolt was always closed when I had the failure to fire. Didn't see any difference in the position of the bolt between having the magazine empty and loaded.

Also, FWIW, I always saw a dent on the cartridge rim, even though it didn't fire.