I just called my dad whom measured one of his 458WM and gave me some imfo.
He said they measure OAL 3.10-3.12 and he loads 300gr flat points which takes trimming the necks "short"* on the 2.5" cases and that a good heavy roll crimp must be put on. And that 70.0 H4895 is compressed "pretty hard"* hence the need for the heavier than normal crimp.
The magazine box will take *(and I'm not sure I understood this but will print what I think he was saying) a 3.15 OAL but was "opened " to do so. And the rails were "worked as well"
* inside the " " are his terms not mine
Like I said I'm not a gunsmith and pay someone whom is when I want work done. He also said that most people don't even know that a magnum was offered in the 600/660 as there were only about 14,000 made. And that not to confuse the 600 & 660 with the Mohawk 660.
Actually, I think alot of people do know about the 600/660 offered in magnums, should be common knowledge to any gun buff over 45 years old or so. They were the 6.5 & 350 Rem. Mags & they were short mags with a Maximum C.O.L. of 2.800" & were designed for these SHORT actions.
This is the WHOLE reason that the 458 2" American was created, to fit a short action because like Esox. said, the 458 is too long & it is not a requirement to be a gunsmith to know that.
Even with the mags "opened up" to 3.15", this is far short of the 3.340" C.O.L. of 458WM ammo. So what you would have is a gun with an action/mag. too short for 458WM factory ammo & you can only use the short for caliber 300 gr. bullet. You and I both know that the 300 gr. is no dangerous game bullet. Obviously, the 300 gr. soft point load would not be as good for big stuff as the 45-70 Garret load that you critical of. Yet earlier in the thread you state that the 458 has such an edge for big animals because you can load the 500+ @ 2,200. Not in this gun!!!! You can call it a 458WM if you want to, but it isn't in my view or SAAMI's either.
You mention a 338WM in these short actions too! That is even worse considering the long, beautiful 338 bullets that you could not use, bullets that make the 338 useful. Even the 200gr. would be seated so deep that the round would be of no logical purpose.