Some of the buffalo rifle competitors have a flexible "blow tube" that they insert into the breech and breathe through the bore between shots to keep the fouling soft. Fairly common technique from what I've seen, but have yet to try it with a breechloader. It does seem to help with muzzleloaders on dry days from my limited experience.
I don't know if I can convince the owner to try BP in this rifle. Nor is it certain that it will end up in my greedy mitts anytime soon, if ever. Depends on whether he gets distracted by something "shinier".
Muzzle end is about pristine. Seems to be a bit of something going on at the breech, but it doesn't seem to have hurt much!
BTW, the chambers on two of the revolvers are tighter than the rifle, and the OAL was set by the Colt Army Special:
So there's plenty of room for fine tuning the rifle from this point. Maybe we'll see more of it...