6mm X 45
both were standardized & I have barrels in each ( the 6 X 45 is a custom barrel ) I'm not sure which would be easier to get dies for ???
BTW... how about 7-30 Waters... can shoot factory rounds with the flat point bullets, but it really comes into it's own if you throw some mid to light weight pointed boat tail bullets in that case... used to take that barrel P Dogging, & had hits out to 300ish yards with a 15" Barrel
BTW... 6.8 SPC might be worth mentioning ( got a carbine barrel in that caliber, though I've yet to wring it out )
In life you either make dust or eat dust...