Hi Andy
I went to my local gun shop today with my Ruger 57 5.7 x 28 - they have a Smith & Wesson M&P 5.7 x 28 in stock. I tried the S& W M&P magazine in the Ruger - it does fit into the magazine well, however it does not "lock" into place since it is slightly longer than the Ruger magazine. The Ruger magazine however does not fit at all into the S&W M&P magazine well since it is too big. See photos below - apologies for the bad photo quality - I took them with my iPhone.
I hope this help.
Best Regards
Paul