Back in my 3 gun competition days, I would shoot thousands of rounds of 223 each year. My competition mags were always USGI aluminum or Thermold 30s. I have actually worn Thermolds (and USGI) mags out after many rounds, but the Thermold 30s are GTG. We rarely left the mags loaded, so I could see potential issues there.

Of all the mags available (including Pmags), I still prefer the standard USGI 20/30 round mags. Haven't tried the high $$$ HK mags and have no intention of getting any.