While it's true that petrolium and black powder don't mix, Vaseline and beeswax, roughly 60/40 or 65/35 worked just great in my ctg. guns and allowed up to 20 shots without any fouling buildup using paper patched bullets and a single grease cookie. Shooting 120gr. 2f in a .45 rifle, usually won't allow 5 shots with any lube, let alone 20- so the lube was working just great.
Some mix neetsfootoil or olive oil, or other veggie cooking oil instead of vaseline and say that works too.
A crusty 'section' only is interesting. This makes me wonder if there is a rough spot in the bore promoting fouling buildup. Normally, a bullet/lube situation will foul near the breech meaning too hard a lube, or at the muzzle, not enough lube, but to foul in the middle are back from the muzzle 8", is strange - has no meaning other than the bore might be a bit rough there. Fouling the full length means an unsuitable lube, as would happen with an alox-type lube modern smokeless lube and black powder.
I shoot patched round balls in my Kodiak. No fouling problems and unlimited shooting without having to wipe. I was going to try bullets, but have resisted so far.
Martin, did you get the lead out of your rifle's butt?