It's hard to shoot the big bore enough to get the barrel that hot because of its diameter, recoil thump and bullet speed. It's probably normal barrel fouling. My 450 Marlin and 45/70 do about 1 1/2 to 2" groups at 100 yards and both tend to walk around a bit after 4 or 5 shots. Personally I want to know where mine is going to shoot under normal hunting situations so I always leave the rifle sling on and always rest it on the forend at the range. As far fouling, you got a choice ... sight it for a clean bore or sight it for a fouled bore. I prefer sighting for a clean, cold barrel so usually clean it after 3 shots and pay more attention to the first shot than the third.