I've been cleaning the copper with Barnes CR10 and am sure all the copper is being removed. Also, When I moved the target to 15 feet, I eyeballed the barrel to be sure I was not contacting anything before the bullet got to target so I know that's not the issue either. The barrel is rifled, but I did consider that it might have a much slower twist rate than is stamped on the barrel, but have not checked that. Would it still tumble even the 55s?