In theory, could the manufacturers not just market a PMAG for example, as a .458 SOCOM magazine? No difference in the mags for this or a standard AR-15 in .223, except that it will hold ten rounds of 458 as opposed to 30 in .223?
I built mine using a Palmetto State Armory upper kit, and PSA lower. No regrets. Did it all for about $680..... Not too much more than an M&P 15 sport with all the things the S&W lacks...ie; forward assist, dust cover & chrome lined barrel