Get thee to a gunsmith! Over the years, many a military firearm has changed caliber. The only way to tell for CERTAIN is to make and measure a chamber casting.
It is a simple process- the rifle is placed muzzle down, the bore plugged with a few patches just in front of the chamber, and a product called Cerro-Safe is melted (very low temp stuff) poured into the chamber, let set up, pop it out and measure with a micrometer.
Cerro-safe is damned near unique in that when it cools, it shrinks enough to remove, but THEN returns to original dimensions. Call around for a smith that will make a chamber casting for you.
DO NOT SHOOT ANYTHING ELSE UNTIL YOU KNOW THE CALIBER. It is possible to have very bad headspace, and to do damage to rifle, self and innocent bystanders.