You probably have a century's worth of layered fouling in there. I'd scrub mine till the patch's came out clean, then soak in solvent overnight, run a brush thru, then have another bunch of black/blue/brown patch's. It took weeks to get down to bare metal. And then you can figure out what will shoot in it. Might have to try some .311s in it before you're done. The foam solvents work really well I've found.
Make sure you've got good OAL, Krags like 'em long. Longer than 2.8", 3.0' even better. Remember too, there were two types of magazine cut offs, they work exactly opposite of each other. Be sure of the one you have, sometimes they seem to migrate to the wrong rifle!