Sixth. constrictions in the bore or rough rifling... fire lap the bore... some say it doesn't work some swear by it. I've done it to a handgun and 2 rifles... in every instance i have AT LEAST cut the grouping in half. no bull, not making up stories, just what has happened from me from personal experience
I always respect people opinion but want to note my experience. But I ruined a barrel with that tubbs firelapping kit. Totally rounded off the rifling. Worst thing I ever did to a gun. My gunsmith and I borescoped it before and after. Yes it smoothed it some, smoothed the rifling off. So beware before attempting not everyone has great experence doing this.