I have an MISR-10 (post-ban) that won't feed properly.  Basically, the bottom part of the bolt that is supposed to catch the cartridge head sometimes misses and scrapes over the brass instead, which pushes it forward but obviously won't chamber it properly.  

This happens with Wolf HP and Winchester FMJ; I've also used Remington SP but not enough to be sure if its a problem.

It happens both with the original steel 10-round mag and with a couple of new polymer 30-rd mags.   It doens't happen every time; sometimes I'll go through 30 rounds with no problem, sometimes I'll get 3 or 4 jams.  I was out yesterday and had both experiences back to back.  I also had it simply refuse, despite multiple attempts, to feed the last round in one of the magazines, a problem I don't usually have.

I keep it good and clean, with adequate oil, so I don't think that's the problem.

I have reduced but not eliminated it by shaving a little off the magazine lips to let the rounds sit higher, but I hesitate to shave too much off because then the magazine won't hold the cartriges in.

The magazines all seem to fit snugly, no wiggle, and no room to move it any further up inside the reveiver.

I was considering looking into new bolt heads; maybe I could find one with a slightly longer protrusion on the bottom.  But even if such a thing exists (and it may not) I don't know if that would fit properly.

Any suggestions?

Thanks in advance.

I think they are US made, but I'm not 100% sure.