Don't get me wrong, the .416Rem is one heck of a ctg., but would not consider one myself. I've always wanted to build up a .404 Jeffery if going to that bore size.
: As well, with top-end loads, a blown '06 case will give about 2,100fps with 400gr. bullets. The base dia. should be in the relm of .470" with a .460 shoulder, similar to a "Gibb's" style of Imp '06 case. Normal IMP '06 runs only .455" on the shoudler with .468" base.
: The case I speak of holds 80gr. of water to the top of the neck. Pretty good for an '06 case, but well under a .404 which is just coasting with 400gr. at 2,150fps. Nice round.