Max OAL for the 6mm Rem is 2.825". 2.820" for the .308. Both with the heaviest bullets Hodgdon lists. Shouldn't need anything but head spacing.
The AI will give you about 100 fps more than the regular 6mm. 6mm Rem AI is a reloading only cartridge. There is no factory ammo or brass.
'U' is not a word.