Re: Barrel Break In
Preparatory cleaning is a must. There's nothing good in there, and what is there (and it's always there with any new barrel) needs to come out before running a bullet down the bore.
What follows is subject to conjecture. I've done both and do very little, if anything, to a custom aftermarket barrel. Generally they don't need it and they are the ones most vulnerable to potential harm.
My efforts to break in a custom aftermarket barrel resulted in discontinuation after about 7 rounds, because I couldn't get the bore to produce any evidence of copper fouling after that.
Original factory barrels tend to be better suited and show more results from break-in, as is defined in the modern lore. Improvements will come in the form if slower fouling accumulation, and greater ease of cleaning. Throat life could be adversely affected.
Greg