You can easily remove powder fouling and copper jacket fouling with various cleaners (I use wipeout/patchout or BTE) but the only way I have found to remove the hard carbon is with an abrasive. I couldn't get it out by brushing with bronze or nylon brushes. It's too hard.
I have successfully used the following abrasive cleaners with one caveat:
JBs Bore Cleaning Compound
Iosso
Holland's Witches Brew
Rem 40X
The caveat is don't use a bronze brush (i don't even use a nylon brush with abrasives either) when you clean the barrel. I don't care what the instructions say. I scratched the bejesus out of my Sako 85 30-06 barrel using a worn out bronze brush with a patch over it and can't recall if I was using Iosso or Witches Brew but I left some obvious scratches over the lands and through the grooves in several spots. Haven't shot the rifle since, so I hope I haven't wrecked its accuracy.
All you need to do is get a jag such as a Parker Hale or Dewey and cut the patch to fit tightly in the bore. (Some folks wrap the patch around the jag like rolling an old school cigarette) Borescope the barrel first to see where the carbon is located and work the patch back and forth in that area. Less is more. Do 10 or so strokes back and forth and then check it with the borescope. If it's clean or clean enough you're done. No need to do any excessive cleaning. You can clean back down to bare steel or just remove enough carbon to make you feel better about the bore...your choice.
What I am finding is it takes 200-300 rounds for the carbon buildup to look significant throughout the grooves and on part of the lands. That's when I will clean with an abrasive. This is using IMR 4350, H 4350 and RL 16 powders. Otherwise, it's just normal cleaning after every range session to get out the powder fouling and copper jacket material out of the barrel. After a typical cleaning session, when I look through the borescope, all I will see in the barrel is black carbon.
How much carbon in the barrel is too much? It's totally subjective or until you start seeing velocity increase by 50-75 fps or you start popping primers, heavy bolt lift etc. with the same load. I don't wait till any of these signs appear. I just clean out the carbon when I think it looks bad.
If you don't think abrasives with a brush will scratch your barrel, you can check out Erik Cortina's youtube interviews with Frank Green of Bartlein barrels. He shows a picture of a barrel a customer cleaned with abrasives and scratched it. I was skeptical myself til i scratched my Sako hammer forged barrel. No more brushes with abrasives for me. They're not necessary. A tight fitting patch with abrasives will get the carbon out.
