gbeecher, not sure which new production ammo you are using, but compare prices with surplus 440-round spam cans of com-bloc ammo - which has been scarce like everything else lately but is already showing up again intermittently at the usual places - just to see if you can save even more than you are already. Cosmoline is correct, Mosins are finicky on ammo, so if you are going to shop around and see what works, look for some 20-round packets of steel-cased surplus broken out of a spam can for experimenting, without spending $100 on a whole spam can.
As for your chambering/extraction problem, pretty much covered above. Two things to add. First, if you really want your Mosin clean, find a way to heat up the receiver (disassembled rifle, of course), then stand/soak it in mineral spirits for a few hours. Several times. (I stand mine in a bucket of boiled water for 10 min., then replace the water with min. spirits and leave it there for an hour, then wipe/clean vigorously, then repeat). Any method will work, if you heat up the receiver. Cosmoline (the petroleum derivative, not the distinguised THR member!) will melt at raised temperatures, and dissolve in contact with min. spirits. Best way to get every bit of cosmo out of every nook and cranny (and if you haven't, examine the area just in front of your Mosin chamber for some impressive nooks and crannies for hardened cosmo to hide in).
Second - are there any scratches on the copper-washed cases? Mosins can have burrs in the chamber that complicate extraction of fired cases, even with a completely cleaned receiver. I have a Tula ex-dragoon with a chamber burr that I'm going to try to fix some day soon (spinning a fired case wrapped with some wet/dry abrasive, there are web postings demonstrating how to do it) - it extracts brass-cased ammo (far too expensive for my Mosin hobby) easily, but steel-cased is very tough. And there are consistent scratches on the steel cases showing where the burr grabs them.
Just an idea.