" ...the Japanese round was 31 caliber, we could not shoot Japanese ammunition in our rifles."
In fact, that is one of the "we...they" stories that happens to be true in a sort of reverse way. The .30-'06 is obviously too long to fit the chamber of a Japanese 7.7 rifle. But what about the opposite?
Normally, those stories are nonsense and can be ignored, but I heard the story that Japanese ammo could be fired in a M1903 rifle from a man who claimed to have done it in combat. I tried it and he was right. Not only did I fire five rounds of Norma 7.7 Jap from a Model 1903A3, but the Japanese clip fit the clip slot of the rifle just fine. I did not check for accuracy but I am sure it would have been good enough in an emergency.
The cases come out with a short neck but otherwise appear normal and with no signs of high pressure.
(FWIW, the 7.7 Japanese rifle bullet is .311", exactly the same as .303 British, and a mere .003" larger than the .308" of the .30-'06.)