I have a new to me Mosin that I took apart today for the first time and discovered a healthy chunk of rust and corrosion on the top of the barrel under the handguard. The bore was dirty but no rust like this. Should I just clean it up and shoot it or be worried? I was thinking of buying a headspace gauge to check it before shooting but am now wondering if it might be a good idea to take it to a smith for a full check. Any info appreciated.