Does anyone know if such an animal exists, and where?

A 25 Round Mag that's compatible w/ the M&P10?

Sure would love to own a few...

vfin

MagPul will have theirs out soon...

I saw 25 round Pmags with the window on a site called epicenterofbattle. they were "out of stock" 20 bucks

Hey why not step up for a 50rd. drum magazine. Go here if you are interested.
High Capacity Magazines - X Products
Mine is like 3 days out from being here and I will give a range report on it when it arrives.

I'm thinking they'd be too heavy and stick out too far. I tried a 30rd mag in my M1A and it was like a 3rd arm.