I shoot a 50BMG.
Everybody who shoots a 50BMG, shoots GI brass... it is the best and the cheapest. I have accumulated about 2,500 pcs over the years, most of it for 25 cents or less, about 1,000 for free.
This one of the more popular 50 cal boards
There is a fair amount of BS there, but it's no different than any shooting forum board. THey all have their blowhards.
Ask for sources there - there are several posters that sell supplies on that board.
Mil pulldown powder is CHEAP and good (~$30 for 8 pounds vs $110 for 8 pounds of H-50BMG)
But H-50BMG is slower, and if wou want the best velocity out of the Hornady 750 A-Maxs, this is the powder to use.