The Magnetospeed three-rounders are a fun concept. If you look at them from their left, you'll note the absence of a mag-catch cutout. It's because the thing is so short it shelves on top of the catch.
The reviews online are not positive. I might be wrong on this, but I believe the company has quietly stopped making this particular insert for their hidey-hold grip. I was curious to see the problem in person, though. I have a Core15 Hardcore lower which sports an abbreviated well. Since this was going to be a deer pistol in 300 AAC, and PA is particularly Fuddian, I thought having no mag sticking from the short well would be less SCAAAARRRRRY.
So, I bought a bunch of the MiniMags, and after looking at the first two it's clear why people are having issues. There's nothing strictly dimensionally wrong with these mags, despite some reviews. The reason comes mostly down to material selection. And the spring selection is probably at fault as well. But mainly the follower is visibly warped just from sitting in the package under tension. There's a reason mags aren't done in non-reinforced ABS (unless you're Kel-Tec). It's not that it couldn't have worked in this application. It's that the time wasn't taken to engineer it for longevity in this form factor with the plastic used.
A straight metal mag chopped down with the floor plate retention lugs reworked higher up the body and a 5-round spring is what I'll eventually try out in place of the MiniMag.