So I have been looking for a way to couple two MPX mags.
I saw the MagCinch, which is sort of old school. I have one on a couple 20 round DPMS 308 mags and they work fine. They make a set for Colt pattern AR 9mm/MP5 mags. But $20 is a lot to throw at a maybe.
So I shopped around some and I found them here:
http://www.mountsplus.com/AR-15_Accessories/AR-15_Scope_Rings/BUFT-MC1.html
Still $20. But here is the kicker... They a coupon code. "cinch" which makes them $1. Really. Then $5 shipping, so $6 total. [they are bound to terminate this little loop hole at somepoint]
I got the first set two days ago. They do take a little fitting to sit right on the MPX mags with all their dimples and bumps. But once they are cinched on there, no movement at all. They are solid. Need to stagger a little, so left mag is an inch taller than right mag.
With the mags setup, and the MPX bolt release right above the mag release, reloads are stupid fast.
I'll upload some pics later.