Modern dimensions for a 12ga call for a minimum chamber diameter of .798" at the forcing cone with a taper of .005" per inch of chamber length. this would give a diameter at the rear of .813" for a 3" chamber. This is "Minimum", add a bit for tolerance & it sure sounds like a 12ga chamber to me. Had this gun been proofed under the old gauge marking system it would have been marked as a 13/1. 13/1 would indicate any bore that would accept a .719" gage but refuse a .729" one. not at all uncommon for a gun with a 12ga chamber.
