Very nice results. What lengths are the barrels used, or were these groups all obtained with your 10" SBR?
I have five .458s now
6", 10", 16" (two) and a bolt R700
Hornady 325 FTX over 29.2 gr of Lil Gun and a S&B LR (YES LR) primer at 2.248" COAL will give you 1,875 FPS and 2 MOA or less WITH MOST 16"+ barrels.
Of course your load may vary and you will need to work up (DO NOT USE THE DATA ABOVE WITHOUT PROPER WORKUP!!!), but I have loaded thousands of them this way for my .458 Socoms. You can push a bolt gun harder.
There are lots of other loads on 458SocomForums
AND... although .458 SOCOM is awesome... .375 SOCOM is even better (I would hunt anything that walks this Continent with .375 SOCOM.
My Smith has the reamers for both if you need a custom... He is very cheap! Drop me a PM if you need more info.