Acen:
I've been using the medium sized rolypoly for a year now, including on all my workup training. You know the loops on the bottom of the Tac-Vest for the gas mask carrier, etc? I took the one on the left side of the vest, immediately left of the closure zipper. I was able to close theclip on the dump bag onto that small bit of webbing holding that plastic loop on. Some would argue that that places it very close to centre on my rig, but that works for me; I have my TCCC (medical) leg bag on my left leg, and I need the dump pouch to be clear of it.
I've found it to work brilliantly for me. Not just for mags, but for isntance when treating casualties, I need somewhere to stash my combat gloves when I put the medcial glvoes on. Likewise somewhere to drop my sharpie that I use for casualty cards, marking application times for tourniquets, and all that stuff. As a 'general-purpose' pouch, it works great for me.
I did cinch up the shock cord just a bit to lobster-trap the bag slightly. I've found it to hold things very well in general though.