You can't stretch nylon or heat it, it will break instead of stretching. If it's a molle pouch, you might get a little bit of space if you are careful in the way you attach the pouch.
If the problem is taking the mags out of the pouch, attach a small piece of rope or ribbon on the molle, when you insert your mag into the pouch it will push the rope down with it. To pull out the mag, pull on the end of the rope that sticks out and it will push the mag out of the pouch easily.
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