RAY: During World War II, American infantrymen carried a rifle that used .30 caliber ammunition. Now, you may ask, what is a .30 caliber bullet? I don’t really know what the arcane measurement is, but it happens that 7.62 millimeters is .30 caliber bullet, or 308/1,000 of an inch.
When hostilities broke out between us and the Japanese, they hurriedly began to make rifles that would fire a .31 caliber bullet, or a 7.7 millimeter bullet. Why would they do this?
RAY: Well, one part of it, I think, is pretty obvious. And that is, if they had to vacate a position and their ammunition were captured, it would be unusable to the Americans. But the other part of it isn’t so obvious. If the reverse happened – that is, if the Japanese captured our ammunition – the smaller-caliber bullet would fit in their rifles, and they would be able to use our bullets to fire on us.
The U.S. .30/06 7.62mm bullet diameter by 63mm case length is 5mm (.2 inches) to long for the Japanese 7.7mm bullet diameter by 58mm case length chamber. Neither cartridge will function in their enemy’s weapons.