A 458Socom,especially in the AR platform,will not produce the power of a well loaded 45-70Gov't. The AR platform will not have near the bbl length needed for more velocity either.
i would also be hesitant to shoot non jacketed bullets in a gas gun also.
You mentioned grizz. That equals a stiff loaded 45-70,>405gr WFNGC (hardcast) ,pushed as fast as possible. Possibly even a 45-90Express. --- SAWMAN