Re: Opinions wanted......
I liked old bear's suggestion of the 6T/CU but with your "cold form" requirement, this is out the window. I have one in a 15" Contender and it is SUPER accurate...taken plenty of g-hogs to 350yds w/ 70gr NBT's.
However...the 6x45 would be a super cartridge. It is tough for me to believe that a 6mm 55gr pill will "out BC" a 55gr .224" bullet...though I have not really looked into this comparison. Have you run the numbers with a 70gr bullet? I think a little less velocity may be beneficial if launching a higher BC pill...as is typically the case.
I've used a Striker 22-250 w/ 50gr VM's reliably to 400yds on groundhogs over here in the east. This rig pumps out the same performance as a .223 Remington w/ about 26" of tube. This would be plenty of medicine for PD's to 400yds.
For your itch to build something a little different then the 6x45 (6-223) would be great. Besides, after tou get that reamer, I have a nice XP-100 that's looking to get spruced up a little!