FWIW, I had a couple of the Thermold 20s for my duty rifle. I loaded them to 18 rounds and both mags failed to feed after riding in the rifles for a few months. The rifles were a Colt M16 and a Colt AR15 carbine. The mag lips spread and would not feed properly. They worked fine when new but if kept loaded in the gun they were unreliable. They would probably work as a range mag if kept unloaded when not in use.

That said, I carried Gi aluminum and Magpul 20 rounders for months and never had a problem. The Thermolds are in the for sale box (with full disclosure) waiting for the next gun show.