If you could only reach 4 1/2" in from the rear end, then chances are the bore was still tapering down some. If you reach in twice that from the breech end, that should give you pretty much the same bore diameter reading you'll get reaching in 9" from the muzzle end. But if the readings you got from the muzzle are accurate, then it's out of proof as a 21, and fairly extensive honing has been done.