This fouling you've described is the worst I've encountered. Right now I've got a gun with two areas in the barrel encrusted with what can only be the grease/BP condition you've described. I may try your gasoline method, though I've found extremely tight patches made of lead removal cloth, and to some extent a little turpentine, to be slowly making progress. But I'm talking hundreds of patches through the constricted section of an otherwise spotless barrel. At first I thought it was lead, but so far it's black dense tar like crud loading the patches. It's the toughest stuff to remove I've ever seen. I've tried every trick in the book, except for full blown immersion; brass screen, steel wool, kroil, acetone, MeK, alcohol, MP7, ammonia, Hoppes, etc, etc.. The gunk in the barrel shrugs it all off.