Magpul Gen M3 PMAG Overview W/ Pics Edit--Review Posted
That's a tough one. Pouches with tight elastic and no stiffening material (a la TACO and "chalupa") require some finesse. If you round off the tab corners, you lose a lot if contact area on the stop, so I'd avoid that.
Here's what works for me:
I insert the magazine back into the pouch with the front corner aimed a little down. As you keep inserting, when the stop catches, twist the magazine along the long axis so that the front edge catches and stretches the elastic a bit and the stop is aimed at the corner of the pouch nearest your body. This will usually allow you to get the stop past the edge of the elastic and insert it fully back into the pouch.
For shingles and the like, I did a "pouch stretching" PowerPoint tutorial for one of the military field test units...basically just wetting and stuffing the pouches and letting them dry. It's worked on even the tightest triple shingle pouches. If anyone is interested, I can see if I can compress it and post it somewhere.