The 7.7mm cartridge is essentially .303 Brit. with the rim removed. The bullets are interchangeable. 6.5 is the older cartridge. After some battlefield experience, the Japanese decided that they needed a heavier caliber rifle, and, as they did with many aspects of their weapons, they borrowed heavily from Western technology.
Links below for those who are curious.
History of the rifle:
http://world.guns.ru/rifle/rfl22-e.htm
An example of said .303/7.7mm bullet for sale:
http://www.midwayusa.com/eproductpag...eitemid=423174