Ten,
I get the feeling that you may not have been bore scoping your barrel throughout its life, but only after there seemed to be a problem. The black stuff that you uncovered was what is called hard carbon fouling, which is much tougher to remove than ordinary powder fouling. Its roughness is what started the copper fouling. Given the extent of your barrel's problem, I would agree that an abrasive was in order, but I will disagree with your choice. Valve lapping compound is usually silicon carbide particles, suspended in grease. For various reasons this is not the best abrasive to use in a barrel. For this use Iosso works well. You can fill up the bristles of a nylon bore brush, that will allow you to reverse the brush and work part of the barrel. The old standby JB bore cleaner is also something to try. Ultimately, the problem may be traced back to a couple of things. A high round count will typically result in roughening of the bore, in, and a few inches in front of the throat, which makes fouling harder to remove, due to its greater adherence to the rough surface. The other cause may be that your normal cleaning routine, was not adequate for the characteristics of the powder and interior finish of the barrel, allowing an accumulation over a large number of rounds, that was not removed by your normal cleaning. Once carbon fouling starts to build, it can be very hard to remove. The trick is to modify your cleaning procedure so that you are keeping up with the problem, while at the same time avoiding damage to the bore. One famous Benchrest shooter used to advocate the use of Iosso on a brush to remove carbon in the first 8-10 inches of the barrel. IMO this was needed because of the powder that he used. Other powder may not have required it. Even so, I have heard that he had been using Carb Out, a liquid carbon remover, with a bronze brush instead of the nylon brush that is mentioned in the directions. In any case, the trick is not to let the carbon accumulate, removing it with each cleaning, before it aggregates into something that is difficult to remove.