Unique is a good bet for special loads in mag brass, along with others listed above. I also know that clays works for cast bullets, along with HS-6, Longshot, 2400, and a several others. Playing with the pressure/velocity gamut by altering powders and charges is much of the fun of handloading, for me. Big bores and big bullets don't need speed to be effective for all applicable purposes, and loads can be tailored to niches, or one-size-fits-all.
Can't beat Unique for this purpose, especially since you have some. Starting loads can be whatever gets the bullet through the bore, up to some fairly good speeds, as it is forgiving in big cases. Once you've gotten Unique figured out, step up to the slower burner.
