I built up a 24" .458 Alaskan on a post M70 action that originally had a .264 Win Mag. 22" bl. = ridiculous.
I put the Shilen tube on it, chambering it for the .450 Aklaskan. I turned off the rims of the .348 cases and rebated them to .532" to fit the bolt face of the m70 action.
J.J. Donnally's Ctg/. conversions book noted the .450 Alaskan ctg. had a 7gr. advantage over the .458 Win Mag. I assume that was to the case mouth. Due to the bottle-neck design, the 'advantage' of the .458 Alaskan might have been greater, but I do not know that.
I used Barnes data for the .458 Win. Mag. but with normal lead/guilding metal 500gr. Hornady bullets as well as Original Barnes lead copper 400gr spitzers and my velocities duplicated what was in that manual, ie: 2,400fps with 400gr. and 2,200fps with the 500gr.
Our own Buckstix noted the .450 Alaskan's capacity duplicated the .458 Win. Mag. I know form my own loading that it was fairly close.
None of my loads in the ".458 Alaskan" cases, were compressed, using 74.0gr. and 72.0gr. IMR4895.
I called this wildcat the .458 Alaskan due to the rebated rim, making it different from the parent .450 Alaskan. Otherwise, it was the same case.
I did not like the recoil, so I duplicated my 2,150fps with the 400gr. This took 8gr. MORE RE#7 than my older .458 2", so I set the barrel back & rechambered it for the .458 2". Thus 51.5gr. Re#7 gave me 2,150fps with the 400gr. Barnes = happy camper.