6.5mm Japanese ammunition...

At the local flea market yesterday, I acquired a very nice Type 38 cavalry carbine (didn't need it, wasn't looking for one, and don't generally care for Japanese stuff), which was just too good to pass-up.
It is in near-perfect mechanical shape and the bore is perfect (really!) - the 'mum has been ground, but the leather sling and cleaning rod are still present.
However... the groove diameter is .268". I know that such bullets are available, intended for the Carcano, but am unsure whether they can be used in the Japanese round, due to the neck diameter of rounds loaded with bullets I am fairly sure are larger than the original Japanese standard and might not chamber freely.
I don't have a single round of the 6.5 Japanese to measure, so...
For those of you who do reload for this caliber:
What was the average diameter of the Japanese military bullet in 6.5mm, and...
Will bullets of .268" diameter, when loaded into the Japanese round, chamber and fire properly?
What has your experience been in loading this round?

FWIW, I have measured a number of other Japanese rifle barrels and am satisfied that a good many (if not all) of them are very much larger across the grooves than their standard bullets - not long ago, a friend gave me a near-new condition Type 99 of early production (chromed bore and bolt face, nicely machined and finished) which measured .317" across the grooves - rather than try to load for that one, I simply made it a present (?) to another friend

Thanks;
mhb - Mike

Metrics are at the bottom. http://stevespages.com/page8d.htm
Hodgdon and Norma agree with Chuck. Bullet diameter is .264".
http://www.chuckhawks.com/6-5x50.htm

Did you cast the barrel to come up with. 268. If so just double check you measurements as common wisdom says. 264 but their are always variences/wear/etc. If you have a large bore I've read you can use carcano bullets. I have reloaded for Carcanos but not Japs so I can't say for sure.

They can vary a lot, some of mine run up to .271. You should have no problem with the .268 bullets, they left a lot of room in the neck for dirty ammo. Just start low and work up. And, despite the larger groove diameter, .264 bullets can give you some amazing accuracy. With gas blowing past through the four grooves, they tend to stay centered during the trip down the bore.

I measured the groove diameter directly (being a barrel maker, I have the proper tools).
They do vary a lot!
I have some .268" bullets on the way, and some .264" types on-hand, but do not like to use under-groove diameter bullets, if anything of proper size is available. My experience is that full diameter bullets work best, other factors being equal.

mhb - Mike

Always looking for another source for. 268,s can I ask where you got yours. Barrel maker, yeah measuring bore would be must have skill set.

come to me through the kindness of a friend, who also shoots 'oddities' - he says his Type 'I' likes them...
Meanwhile, I'm looking for some more of them... and some brass.

mhb - Mike

FWIW: Barrel makers spend quite as much time contemplating the insides of their 'holes' as proctologists and gynecologists do on their respective orifices and associated plumbing...