You could take 100 top level shooters, half who "break in" their barrels, half who don't. You would end up with 100 top level shooters with no correlation to some mystical cleaning break in. If your barrel is rough, a bore brush won't do anything. My experience is jacketed bullets won't do anything. Maybe thousands of round will smooth a barrel. If you want to smooth your barrel in a matter of hours or days, the only effective option is lapping.