Not saying anybody is wrong, but that bullet could well be from a .30-06 round. My family handloads most of their ammunition, and we have used both .308, and .30-06 "Ball" ammo just for the Brass. Grandpa bought several thousand rounds of '06 Ball ammo (in Garand clips) back in the 50's.
I used his '03-A3 for Deer Hunting when I was a teenager and pulled a lot of those '06 bullets to reload them with modern primers, powder, and hollow point bullets. The base of the "pulled" bullets looked just like the base of the bullet in your picture, (rolled copper edge and exposed lead base).
The ogive doesn't look quite the same, but its kind of hard to tell with it being so badly mangled. How many Land marks are left from the rifling? .03-A3's and Garands, from what I can remember, both had four groove rifling, which would in turn, leave four Lands. I don't know what kind of rifling the American Machine guns used, or what the Japanese used.
I've heard that later production Japanese rifling was only two-groove. My Grandpa has a 7.7 Arisaka, but I have never paid any attention to the rifling. Next time I see it, I'll check and see if that rumour is true. It is a late production model. Milling marks all over the steel parts, and the stock looks like it was finished by a team of beavers...