What has my Mosin Nagant M38 been fed?

Unfortunately this is the result of using corrosive ammo and not cleaning your gun meticulously! It won't necessarily have ruined the rifle, but it will reduce the accuracy a teense. I've had, and do have several rifles that have suffered this fate. I wanted them anyway, and am quite happy with them the way they are! I have an Enfield #4 MKI that I had a tough time even getting a 30 cal cleaning rod to go through the bore, when I bought it! It shoots fine and is pretty darn accurate! This rifle also had a depression eroded into the bolt face where it had corroded also. I had that built back up and rehead spaced, as I didn't quite trust it the way it was. I'm not sure what the brown flat flakes are. They where also in this rifle! I don't think I'll ever see them in this gun anymore, though! You have done basically what I did, clean the crap out of it, shoot it and clean it again. Now you can enjoy it! :)