Blackhawker , I've used both S.R standard and mag primers in my loads , I've seen no difference at all. My heavier load consists of W-296 and Horn 240 gr XTP-Mag bullets. Gets me decent accuracy and lots of "knock out power". I've been told by the Horn people not to use the 250 gr XTP in a 454 due to the thickness of the jacket , they're afraid of separation due to the pressure. It was designed for the 45 Colt.
Last year I worked up a load using 2400 and the 240XTP , loaded down somewhat , it shoots very close to my "heavy loaded" 45 colt , as far as poa is concerned. I wanted to be able to shoot both the 454 and 45 colt through my scoped SRH with out changing the scope adjustments. I pretty much accomplished that. The 454 calculates out to 1200 Ft /lbs....where as the 45C works out to just under 800 FT/lbs. Just for reference , my 44 mag load works out to 1125 Ft/lbs. That's all with 7 1/2" barrels.
My 44 mag load is 24.5 gr of w-296 under a 240 XTP. Good luck , regards ,irold