Gottahaveone is correct.
A few other things to consider to help you rid the receiver of cosmo. Stand the receiver chamber-side down in a bucket of hot water to heat it up, then replace the water with mineral spirits (a good solvent for cosmo). Repeat. If there is much cosmo being melted off, you will see it as a slick in the water/spirits.
Also - I am convinced there are causes of sticky bolt other than cosmo fouling. Poorly fitted bolts, or extractor problems - even burrs in the chamber.
To the OP - do your fired shell casings have any scratch marks near the rim (consistently, with all of the casings)? If so - chamber burr is possible. One way to check is to alternate steel-case surplus ammo and some brass-cased (Winchester, Sellier-Bellot, or Yugo non-steel surplus). If the steel sticks, and has scratches, and the brass extracts normally - you may have a chamber problem (I have this situation on one Mosin). Solution: Mosins are cheap, get another! In my case unfortunately it's a beautiful and less common receiver that has the chamber problem, so I might try to continue to fix the burrs or just keep it as a brass-only rifle.
Does your bolt cycle easily when the rifle is cold and empty? My most recent acquisition simply has a bad bolt. Never easy to work right from the start, gets downright difficult as you cycle it cold and empty. On the range - forget it, locked up after about 10 rounds. (This rifle was cleaned to death first thing upon arriving from UPS - cosmo not the issue)
If you have another Mosin, try the bolt in the problem rifle (don't actually shoot it that way unless you have/can borrow a headspace gauge to first make sure the bolt is safe to use with that rifle). If another bolt works much better (as with my new rifle with the bad bolt), then it may be a bolt/rifle match issue.
So try gottahaveone's approach - and maybe back it up with some heat and mineral spirits, as I suggested - and if that doesn't work, consider what I've written above.
