Been fooling around with a 7.7x58 Type 99 that my dad sporterized years ago. I don't know why he did it, other than boredom -- which is the same reason I'm messing with it. It slugs .319 with .303 bore near the muzzle, which gets larger near the chamber. The chamber neck will take a .348 plug gauge.
It doesn't shoot worth a hoot with .312 jacketed. I'm considering trying 8 mm cast bullets in it. I have some brass with .010 neck thickness, so I think I should be safe with .323 boolits.
I reformed some 8 mm brass for it using Unique and buckwheat, and it cleaned the bore up to the point that I actually get white patches out of it eventually.
Anybody have any similar Jap tales or advice?