In a SBH like yours, with a 250gr cast bullet, I figured 22 grains of S265 was about max. Flattened the primers nicely. Gave 40mm at 25m and 1330 fps 6 shot average.

If you want to go heavier, 13.5 grains of S221 behind the 300gr Frontier gives me about 1100 fps and it's prolly a grain, grain and a half maybe from max.

That's about the only data I have that might be relevant.

This is all with CCI 300 and mixed cases. I don't think you need magnum primers and you can't use rifle primers, that only works for small primers.