I agree with lemoyne. You never know what a rifle will like. I would shoot some Pyrodex RS in loose form. Start at 100 grains and then work up. If it is still holding good with 120 grains of loose you should be just fine for your magnum requirements.

Swab between shots. Pyrodex produces a nice soft fowling. Granted it is dirty, but it is consistant. If you want to shoot Triple Se7en I think you would have good luck with that powder as well.