Michael, yes sir, the M&P® FPC is compatible with M&P full-size and compact pistol double-stack magazines. So this will fit and function just fine. Hope this helps.

Why can't I find 44 Mag. Ammunition

Hi Michael and welcome. Yes Sir it will. The FPC was designed to use M&P full-size and Compact pistol double-stack magazines. This drum has the same Magazine tower and will work no issue. Cheers and stay safe!

Michael, yes sir, this will work, the FPC is compatible with M&P double stack magazines.
Stay safe & have a great day!