Leatherneck,
Semper Fi!

Can't help with the 40S&W with cast in the M&P ------ yet, but, I did load for a Browning High Power for a bit with cast bullets.

I can only say that it is like any other semi-auto caliber if it is shot out of a revolver or a fully supported chambered semi-auto, as the M&P is.

Still, I would use my rule of thumb for this caliber and NEVER use anything faster than Unique in it for top end loads and W231/HP-38 for light target loads. There are LOTS of other powders out there, I am only speaking in terms of burn rate. For SURE, nothing faster than W231/HP-38 for me though.

Loading for this caliber can be quite rewarding. Be safe and have fun!