The SOCOM does not need special mags. Some of the later generation P-mags will limit your OAL if you use pointy bullets but aren't any problem for the various 300 grain .458 HPs. I like Lancers the best. My mags will load 4 rounds in a 10-round 5.56 mag, 7 in a 20, and 11 in a 30.
If you decide in favor of the .458, think hard about a carbine-gassed barrel. H110 is a very good powder for it, but it can get finicky about cycling or lock-back in a mid-gassed barrel. I use an adjustable gas block on mine to use the slower-burners if I want to, but have been shooting a lot of 350 grain Beartooth Piledriver Lites lately and seem to be getting the best velocity with H110. RL7 and H4198 are great for the heavier bullets and you could use mid-gas, then. I like the 300 to 350 grain range of bullets, but can use the heavies and slower powders with the adjustable gas block.
Brass for the .458 is expensive, but doesn't seem to stretch. Avoid the Lyman small base die like the plague, and if loading lead, use either a Lee Powder-through die or a Lyman M-die (preferred) to flare, then use either a Lee FCD or Lyman Taper-crimp die (again, preferred) to iron the flare out. I have not personally found it necessary to crimp.
I don't have any experience with any of the other big-bore cartridges, so can't comment on them. And, mine has a 16.25" Tromix barrel.
