I discovered that the face of the receiver on US krags isn't square when I rebarrelled one with a larger diameter barrel that had a flange.  Receiver should still be ok, looks like most material came off the edge of the barrel.  I'd rent a 30-40 reamer and make use of it before rebarreling, if it has good rifling.  It's weird that a 30-40 doesn't fit, why modify the edge of the barrel if what you're trying to feed doesn't fit the chamber?  Maybe you have something in the chamber, a bit of old caseneck or something like that.