Hi- and welcome to the forum, When you get a minute, drop by the intro thread and say howdy.
As the man said- odds are you have a 7.7 Arisaka. HOWEVER-
Many war bring backs were sporterized- which went way beyond restocking and adding a scope- it some cases, rechambering. The only way to POSITIVELY tell what caliber it is NOW in, is to have a gunsmith do a chamber cast. Great product called Cerro Safe. fairly quick, easy and cheap.
Depending on what has been done to the rifle, you may also want to think about restoring it to original configuration. An intact mum at the chamber generally indicates a captured weapon, not a surrendered one.
In the meantime, some reading for you. Explore all of the tabs, and the assembly/ disassembly module on the right side of screen,