From the mouth (or fingers) of well known barrel makers...I don't know...I think they know a bit more about barrel and barrel cleaning them 99% of us out here. Take it for what its worth:
Chris Dichter, Owner, Pac-Nor Barreling
"My preference is cleaning every 20-30 rounds. We settled on the Witches' Brew from Dave Holland, which is 1200 micron-inch aluminum oxide in Kroil. It removes anything that isn't gun-barrel steel. It gets carbon and copper out very effectively. Be careful with JB and abrasives--A JB'd patch around a bristle brush can become a lap more than a cleaning agent."
John Krieger, Krieger Barrels
"I've been asked about the abrasive bore cleaners. My advice is to proceed with caution. Use JB sparingly. A JB-coated patch becomes more of a surface lap. I've seen people actually lap a taper in their bore using JB on a patch. You want to clean your barrel, not change its dimensions! I'd just put JB on a bristle brush if you feel compelled to use it."
Gary Schneider, Schneider Rifle Barrels
"For the carbon build-up in the throat area, I don't think you need to treat that differently than fouling elsewhere in the bore. I discourage the use of abrasives. JB is a 1200-grit lapping compound. When you use it aggressively, some of that black you see on the patch is barrel steel. JB and Rem-Clean can remove steel and, in my opinion, used to excess, these products will decrease the life of the barrel. Yes I know Tony Boyer uses lots of JB. But he keeps a large inventory of barrels and he has no compunction about wearing out a barrel. Most shooters demand longer barrel life and hence should avoid using abrasives."
Bill Shehane, Two-Time IBS 1000-Yard Shooter of the Year, Owner D&B Supply
"For normal cleaning I use regular Montana X-Treme liquid solvent, with Montana X-Treme Bore Creme every other match or so (100-150 rounds). I use 50 BMG for break-in or if I have a barrel that is giving a problem with copper fouling. (Using the 50 BMG during a break-in procedure, after 15-20 rounds the new barrel is ready to go.) I used to use JB pretty religiously every 100-150 rounds. Now I use the MT X-Treme bore cream in a syringe and it works just as well as the JB and it's not as abrasive. If you use it in the throat area every 100-150 rounds I find you can keep the carbon build-up under control."
Michelle Sutton, Co-Owner, Hart Rifle Barrels
"I've also personally witnessed the damage that can be done with over-aggressive cleaning. I had one barrel that I cleaned using Rem-Clean on a patch over a brass brush. (Rem-Clean is an abrasive similar to JB.) I made 10 round-trips through the bore, running the brush just a bit past the muzzle on each cycle. Then I went out and shot the gun. The groups were terrible and the last two inches of the barrel was packed with copper. I inspected what had been a nice shiny, sharp crown and it looked like it had been bead-blasted. I then measured the bore diameter at the crown and found that the groove dimension had increased .0002" (two ten-thousandths). I ended up cutting a half-inch off the barrel and recrowning it, but the barrel still shot very poorly. It basically had been ruined for competitive purposes.
Chris Dichter has observed that JB on a patch can work like a lap. I personally saw how Rem-Clean on a patch can behave similarly."
