Thermolds have two very serious problems:
- In cold weather, the plastic becomes brittle, and dropping a mag or flopping down on the ground and hitting the floorplate will often break them, resulting in your ammo pouring out the bottom of the mag.
- In dusty conditions, grit buildup will lock up the follower. Because both the follower and the mag body is plastic, this problem is much much worse than on a USGI mag, as dust won't inbed itself in the aluminum like it does the plastic.
Now, neither of these problems are likely to be a major issue at the range, which for most folks is fairly light duty that doesn't push their weapons system much. At a shooting school, or even worse, in actual combat, their shortcomings become far more evident.
That's why both Canada and Israel abandoned plastic AR mags for USGI, even though in both cases the plastic mags were domestically manufactured and much less expensive. Neither country puts their own people out of work to pay more for a competing product for no reason, so it should be easy to figure out how strongly they felt the need to do so.
-Troy