I'm also going to chime in and say that it's probably a combination of ancient cosmoline in the chamber and lacquer-coated ammo.
It's
possible there's a burr in the chamber; however, the interruptor on a Mosin-Nagant tends to scratch the shell casings even if everything is fine. Could you post a picture of a fired casing? I've dealt with a minor burr before; fixed it with an improvised chamber reamer made from old shell casing wrapped in 600-grit sandpaper and screwed onto the end of a steel rod.
