I've got a sweet little .25-20 pump. I've also got a line on a bunch of .251" bullets (35 and 50 XTPs and 50 lead FN) from a buddy who got rid of a .25ACP.

Can those bullets be reasonably paperpatched up to work in the .25-20?

I'd say more then likely.

GTC

I never had much success paper patching jacketed bullets, because the rifling tended to cut the paper instead of forming it into the bullet like it does with lead bullets. YMMV

It definitely can work well with lead bullets of course.

I hope you have great dexterity in your fingers!

Have you considered a simple swage die to bump them up a few thousandths?