Oh, you never know. One of the M11/9s I bought back in the last decade came with a couple of converted pistol mags. They looked like some kind of aftermarket mag for a Browning HP or S&W pistol. I say they were aftermarket, as they were 20-round capacity, and unmarked. They had spacers and a mag stop welded on front and back to fit the M11 magwell. Both of them ran just great, whether in stock high-speed mode, or with the gun in slow-fire mode.
I don't use them much because of the limited capacity, and the mag stop tends to wedge in place with the magwell that I welded on a couple of years later. They fit and function fine, but you have to pull them to get them out.
I ended up selling one when I sold one of the guns. I've got the other one around here somewhere.
That being said, I've never heard of Mega Mag brand. I'm not going to run out and buy a truckload, but I'd be interested to hear how they work out. I see they list TEC-9 mags by the same company as well. Who knows - if the M11 mags get a favorable report, I'll have to try a couple for the jam-o-matic TEC.