Jerry,
"Europeans often label .25 caliber cartridges (.250" bore and .256-.257" groove diameter) "6.5mm," which is actually .26 caliber (.256" bore and .264" groove diameter). The metric nomenclature for the .25-35 WCF, which uses standard .25 caliber (.257") bullets, is 6.5x52R. True 6.5mm cartridges, such as the 6.5x55 SE, use .264" bullets." from the site Cartridge Names This sounds like what you are running into. As SDH suggested, cast the chamber and throat. You could also drive a piece of buckshot into the bore and mike it. I use a wooden dowel for this when necessary and only drive it deep enough to get a swaged cylinder and then back out with it. Buy some cerrosafe (sp?) and cast it. Perhaps a 250 Savage or 257 Roberts since the 57mm cases are chambering, I lean towards 257 R.
Thaine

It ain't ignorance that does the most damage, it's knowing so derned much that ain't so! J. Billings