If I could see the top of the receiver clearly, I could tell you if it was origonally a Type 77 (7.7 Jap) or Type 38 (6.5 Jap). As someone else said, it could have easily been rechambered though. At one time it was fairly popular to rechamber the 7.77's to 30-06. Although the weren't very accurate since you had a .308 bullet and a .311 barrel. The only safe way to tell the caliber is by a chamber cast.