Check the head space...
Check the head space...I second this recommendation. A head space issue can be a safety concern. One sign that this is the issue is if the spent cartridge is bulged out at all. Does it slide easily in out of the chamber, or once fired will it not chamber like an unfired cartridge. Also, check the extractor.
And be careful when using yellow tip "heavy ball" ammo. It's meant for machine guns and the only way to eject it from a Mosin is with a 2x4.
And extractor/spring.