Re: .458 socom rounds?
Don't have the .458, but I do have a .50 Beo, and it will use some GI mags, most in fact, but the odd one may need trimming/filing. They have mags for these rounds, and I bet the .458 would work in them too. Only difference is wider feed lips and they have a different follower. But regular followers work and you can mod most mags by just fitting them and filing a bit on the feed lips. The ones that WILL NOT WORK are the kind that have ribs INSIDE the mag. For instance, I got two cheap 42rd. polymer mags. Promag. The mags are actually not all that bad for the price, but they have a rib that prevents the loading of any single stack rounds. So bear that in mind, that is at least one type that won't work at all.
Anyway, my reason for chiming in is to agree with Saito, if you aren't a handloader these calibres may not be of much use to you. Ammo is very expensive for these and rare as well, and not all that cheap truth be told to handload --and from my experience, the brass doesn't last as long either. At least in .50 Beo, it is a stiff one indeed.
.50 feeds fine, but I bet that .458 feeds a lot sweeter.
Big bore AR's are a real blast too! So much so I've damaged one of the older collapsible stocks. Plan on using a B5 collapsible later, sort of like the SOPMOD? Much tougher, better suited. Fixed stocks are actually recommended for many of these big bores and they feel a lot better with one, with a rifle length buffer, say an A1 stock.
Good luck!