OK, here is why you need to break in a barrel. When you machine metal tiny burs are left on the metal. There is no modern machining practice that doesn't leave these burs. Even if the metal is polished the burs might be microscopic but they still exist. Cleaning your rifle a few extra times will remove these burs faster than just shooting it.
Yes, a nylon brush is more abrasive than a brass brush. If any of you do a lot of fishing you have had to have new eyes put on on your favorite fishing rod because the monofilament cut into the ceramic liners of the rod eyes. This ceramic material is harder than a rifle barrel. The brass just bends or breaks if you use an oversized brush. A nylon brush just bends and keeps applying the same pressure.
For you Mosin Nagant owners: If you ever use a drill to clean a rifle barrel, try to use a half inch heavy duty drill. A 3/8 drill is much faster than a half inch drill. With a half inch drill you can use it's power to slowly spin the brush to safely clean/smooth the barrel.
"I do not aim with my hand; I aim with my eye. He who aims with his hand has forgotten the face of his father.
I do not shoot with my hand; I shoot with my mind. He who shoots with his hand has forgotten the face of his father.
I do not kill with my gun; I kill with my heart. He who kills with his gun has forgotten the face of his father."
Adopt a pet!! http://www.aspca.org/
Some of the finest pleasure horses come from here: http://www.canterusa.org/