Themauserkid,
There is bad news and good news. The bad news is there are several distinct versions of 9.3x72R cartridges. The good news is whichever version it is, we can figure it out and get you shooting. Based only on your description of the gun and your unsuccessful effort to chamber a cartridge, I believe the rifle is chambered for the "D" (Deutsche) version and the cartridges you used were likely the "N" (normalizert?- standardized) version. There are a couple ways to deal with this, but we need more than my guess, to be sure. We need to start with clear legible photos of the proof marks (if any) on the underside of the barrels and will likely include making a chamber cast, as you suggested.
Mike
