Nice finds! The ring is not part of any type of rifle round that I know of. But I am not an expert on Japanese ammo either... who knows what those silly Japanese did back then! The ring does resemble an outter race from a bearing, but I would think that the race would be a hardened steel. I don't think that copper would make a very good race because it would rapidly wear out. Sorry, I'm no help on this one other than to say I know it didn't come from a US rifle round nor a hand grenade.
By the way, if the 9mm rounds are old, they would be .38's unless you found them near Japan! The Nambu Type 14 which was introduced in 1938 was a Japanese officer's pistol and seeing as how you were finding quite a few, I doubt that there were that many officers around in the middle of battle. I guess I should stop here.... before I dig a really deep hole that I have to climb out of and ask a question. Where did you find these?