Lots of .25-20s were ruined by the use of corrosive ammo, it is a well known problem with the older guns. Hopefully Dr A's just needs better ammo or a good bore cleaning.
I have a BF falling block pistol in .25-20 and it is a tack driver; BF used to make a single shot rifle in that chambering, maybe they still do. With a Simmons 2-7x and Sierra 75HPs it will agg into 0.8 moa. If you want a quiet small game round it is a lot better than the .256 WM or the .218 Bee, those have a lot more crack unless loaded way down, and that means you'll be reloading for any of them for best performance.
My 1894CL likes the Speer 75FNs, shoots them very well and they are great killers on rabbits, grouse, etc. The flat nose of that bullet transmits shock well and kills small game great, far better than a .22 at higher velocity IME. Loaded up to 1900+ fps it would work on deer, but IMO headshooting deer is a fools erand. Seen far too many deer with jaws shot off, eyes shot out, all from macho men who thought thye couldn't miss a tiny target. Yes, my old man did it in the 40s to feed the family, but I have no idea how many got away wounded..... :evil: