For a fun load for your M&P, you could do no better than good old Unique, It's a little bulkier than some and fills the case up more, and doesn't have the quick pressure spike of the target powders. I use a Hornady powder measure with the bushings, same principle as the Lil Dandy, and Unique works very well. If you want a more economical powder (more shots per pound) try a quicker target powder like HP38, W231, AA#2, they all meter exceptionally well. Iv'e sent thousands of 158gr slugs out of my S&W's using Unique, and it's hard to beat!