I tend to believe that BBL break in is bull, even on mass produced bbls I don't believe you're honestly going to help your accuracy much by doing it....BUT, I do it still, better I do everything I can to help my accuracy, even if it doesn't help, at least I did it if it would have. Kinda like not being superstitious, then having a bad day on Friday the 13th, worrying about breaking mirros, or not walking under ladders.

There are going to be a million different ways that people will tell you do do this, basically none of them are right, and none are wrong.

Breaking in a bbl basically consists of filling "pits" with copper and smoothing the grains of metal in the bbl, getting everything to line up the right way (that's why you're never supposed to run patches through muzzle to action too, so you don't alter this at all)

Lots of varminters and target shooters swear you should use some sort of polishing compound in your bbl to help smooth things out, I'd tend to agree with that, but I'd fire a box or so before adding the polishing compound, to kind of get things going so to speak, get things pointed the right way, then smooth it out.

My method is usually run 5rnds through cleaning to CLEAN after each one, then run 3shot groups for 3-4groups cleaning after each one (fire them fast enough for the bbl to get warm, not hot though), then I go to cold grouping at 3 shots each, running a solvent patch and an oil patch through each time.

Pretty much the only thing I honestly feel break-ins do is get things shooting to their potential faster, run enough rounds through your gun and it's going to get to the same point after a while, it just speeds things up to give exposure a raw bbl for a while.