The only 500S&W Mag anything that I have ever owned was a Handi-Rifle in that caliber. Punishing is not the word for full factory loads in that thing. Probably why it isn't around anymore!
At any rate, I chose to use Trail Boss in this enormous case. I couldn't find any loads so I called Hodgdon. Here is what they told me: "Mark the case where the base of the bullet will sit when seated properly and fill the case up to there with Trail Boss, that is the maximum load. Reduce by 10% to get a minimum load."
I found out that I didn't need a minimum load!
Cream puff is the operative word here!