I'm assuming you know how to do a complete tear down. You need to hone the chamber to get rid of a fine rust that forms because there is no really good way to clean the chamber area. The little bit of rust and sometimes tiny pits cause the cartridge to stick just enough upon firing to mess up the timing, sometimes ripping off a piece of rim, or rarely ripping off a chunk of the bolt. Polishing the chamber cures the problem. Goat.
Edit: changing the recoil spring is a good idea on general principles. GW