I have two barrels I pulled off donor actions, a old Mauser and an Enfield P-17.

Both the bore look the same, the rifling is there but everything is coated gray and there seem to be tiny lines perpendicular to the bore, going over the lands and grooves. I have been brass brushing with Kroil & Shooters Bore Scrubber for a few days now and gray sludge keeps coming out but the bore hasn't improved a whole lot.

Is this carbon fouling? Am I wasting my time or is there a chance there's a good bore under there?

Try some Rem Clean or J&B Bore paste.

I would suggest a sparing use of a stainless steel brush.

kerosene and boiling water will loosen any accumulated crud.
rob

Try some military bore cleaner, smells like shit, works good on carbon. I picked up some 2 oz cans years ago for 50 cents each, I don't know what's in it, but it smells like Lysol or creasote and gets the black gunk out fine.

Marked ,Cleaner-rifle bore, spec USA 2-117, 44-700.6, SC Johnson and sons Inc