Same powders you would use for max loads in the 357 and 44 Magnum, just less of it due to the smaller case. Cleck Alliance and Hodgdon's websites. As long as you shoot them in a Magnum revolver, and if you have non-magnum revolvers, be sure you mark the reload box clearly that they are Magnum pressure loads. I would also paint the case heads with a colored magic marker to identify the loads when out of the box. Elmer Kieth and the early work on the 357 and 44 Magnum were all done with 38 and 44 Special brass, the factory lengthened the brass only for the prevention of chambering in a revolver not made for the pressure, the case capacity was not an issue.