That's truly sad that someone paid that much for a muzzleloader and neglected the barrel! I know I check all my ML barres several times a year and my Encore gets checked monthly....just to make sure. I've been using bore butter in mine from day one as new. I also clean mostly with ispropyl alcohol and it works fairly well so far. Clean it as well as you can. Maybe an oversized bore brush and get all the RUST out. Then shoot the heck out of it using bore butter. The bore butter should season it so you don't get the continuous easy fouling and it will be better for loading and cleaning in the future. A far as accuracy, if the rifling is intact, I'd say you're ok.
How did you do on the prices? I bought a 45-70 pistol barrel for mine used and it was a heck of a deal! Heck of a bark too! I'm shooting the Hornady lever-evo-lution's in it and it's easily a 150 yd pistol with the 2-6 variable scope on it....if I can hold it without flinching from recoil! Not really as bad as you think though. I also have a 12 g rifled barrel for mine. If going that route for rifled shotgun barrels, I strongly suggest the 20 g verses the 12 g. If you use the 12 g , you have to pull the rear sight to mount a scope because you have to use the sight screw holes to mount your base. 20 g is on the top of the barrel soyou can keep iron sights and use see-thru rings if wanted. Good luck
