The other week when I was Range Officer a guy shooting a Mosin had it jam up and wouldn't open the bolt after firing surplus ammo. I was called over and yep it was jammed. I asked the guy if he wanted to pound on it and he said go ahead. A hard strike with a gloved hand opened the bolt and cleared the fired case REAL well. He didn't realize that he wasn't using a rifle that sometimes needed to be talked to. The rifle was good as new afterwards.
That'll happen with a Mosin that hasn't been cleaned well. The Russians stored these rifles in cosmoline, which needs to be cleaned out real well when you buy one.