I have never seen a short range benchrest shooter take off a brush at the muzzle. Barrels wear, including crowns, and it is a good idea to refresh the latter from time to time.
Some time back, someone published some very high magnification pictures of the edges of barrel crowns that supposedly showed wear from brushes being pulled back into the barrel. The problem that I had with those pictures was that the what they showed extended past the reach of a brush, so they must have been caused by something else. My guess is unburned particles at the moment when the heel of the bullet cleared the crown.
A friend of mine, who is a top level service rifle gunsmith, once showed me something on a high round M1A barrel. HE dry brushed the barrel a little to remove most of the powder fouling from its last shooting session, so that the slight haze of jacket fouling was visible, at the muzzle. Your could see a very short segment of the bore, just behind the crown, where the copper wash had stopped. He told me that this indicated a slight enlargement from wear, caused by hot gasses and unburned powder blowing by bullets BTs just after the bullet shank cleared the bore. When he saw that, he would shorten the barrel a quarter inch and recrown it.
As far as brushing and hard carbon goes, if a brush took it out, it was ordinary powder fouling. Hard carbon requires careful use of abrasives, and may or may not be a problem depending on the powder and perhaps the cartridge.
I believe in using the least aggressive cleaning regimen that gets the job done without too many rod strokes. I also believe that most shooters have no idea what is in there bores, because they lack, or do not have access to a bore scope. Without that, we are all just guessing.
One common mistake is to think that because a patch comes out white, that the barrel is clean. Real, baked in, hard carbon will show a white patch, after everything else has been cleaned out of the barrel. I cannot tell you how many times I have seen someone chasing his tail on accuracy issues only to find out, upon inspection with a bore scope, that his barrel was not clean.
Fellows think that they are saving their barrels by cleaning ineffectively, but the number of rounds that I have seen them shoot trying to reclaim accuracy by tuning, when the problem was carbon, put a lot more wear on their barrels than proper cleaning would have...a lot., and cost them a lot of gasoline, time and components as well.
I am extremely careful with abrasives, and use them as little as I can get away with, but sometimes they are a necessity. One exception has been 133 in my PPCs. It can be kept up with with bronze brushing, patches and solvent. There may be others that I have not tried.
Take a look at the gouges on the hulls of boats caused by that nylon line rubbing against them when your fish decides to go under the boat.
