I've had very good luck with FedOrd and PMI 40-rounders, which were made on the same pressing / tooling (FedOrd owned it first, then it went to PMI). The steel is thick enough that it doesn't flex. I have one particular mag I've owned for more than a decade and must have emptied hundreds of times, mostly in full-auto dumps. Still runs flawlessly, as do the less-used ones.
Every other 40-rounder I've seen or tried -- USA, Western, etc. -- was stamped out of thinner steel on different machinery. First off, the geometry is off; second, the sides flex in and out, and when full they swell out so fat they simply won't feed.
Still, while PMI/FedOrds are fun and reliable range mags, I only trust USGI for serious work.