I have to add my .02 to this discussion.
"I have heard of absolutely terrible barrels which, somehow, still shot a perfectly good group."
I can believe that. Two examples....I have a little British Bulldog pistol that I picked up at auction. The barrel was and is so terribly pitted that the rifling is barely visible and then only in spots. I did the steel wool thing with various grades. I soaked it in Evapo-Rust. I did the steel wool again. It should not shoot, the bore is damaged but, y'know that little gun puts all of it's shots right where it is pointed at five yards - about as far as one would expect to shoot a little .38 Short. No tumbling either.
Number two: I also have a third hand (at least) Lyman GPR, a flint gun in .50 caliber. The bore has pits in it; that is the way it came. NOT horrible but they are there. The gun NEVER gives me a clean patch when I'm done shooting. You name the cleaning method, I've probably tried it. Twenty, thirty patches...there's always some grime.
Point? That rifle gives me 1 1/2" groups at 100 yards anytime I try.
Do clean up the barrel the best way that you can and give it a good test before selling it out as lost.
Pete