I use the Ramshot powders quite a bit. Magnum is their slowest powder, slower than IMR 7828, and just a titch (technical term) faster than RL25. It's a very fine ball powder that meters like water, great to use on the bench. It is relatively clean burning and supposedly not very sensitive to temperature extremes. It is made in Belgium and is imported and distributed in the US by Western Powders in Miles City MT. Check out http://www.ramshot.com/powders/
they have a pretty nice loading guide online. Hope this helps a little.