In the case of abrasives in the barrel, a little goes a long way. I got a new Ruger MKIII 512 that didn�t shoot that well. I suspected a little residual roughness from the manufacturing process. I hand lapped it with JB Bore Bright. Six passes with the stuff. Not much, but enough to take out the roughness. It shoots much better now.
I have some experience with this. I have ruined a couple of barrels in the past by over-enthusiastic use of abrasives.
