Re: 6.5 x .250 savage load data
Ahh... I wasn't even thinking Arisaka this morning. Was pre-occupied with it probably being a euro surplus rig (M-S, Carcano, etc)
The rechamber job was probably done to find a source of brass since 6.5x50 is still a difficult one to find brass for.
I would suspect that even with the minor case volume differences you could use 6.5x50 data. The parent .250 is a bit shorter but also slightly fatter than the Jap case. I would think internal volume would be within a couple grains or so of each other.
Have you compared your shells to the dimension charts in a reloading manual? Is the body of the case about 1.637" (to the base of the neck) like the .250 or is it closer to 1.709" like the 6.5x50? That small bit of length will make the difference in internal volume. If it's the longer dimension, I think 6.5x50 starting loads would be a safe starting point...but if it's shorter like the .250 even the starter loads could give you high pressures. Another 10% reduction in starting loads would probably be prudent...I can't guarantee that it would be safe though.
I would also suggest sticking with a slower powder for that particular case volume since you'll probably run out of case volume before you hit a max load. I should emphasize the probably...but that's usually a general rule.
Loading for a wildcat can be a tricky process if there is no similar cartridge to cross-reference data to, so always exercise caution. A chronograph and a careful eye for pressure are must-haves.