IIRC 2nd generation P mags are pretty much the upgrade replacement for metal AR mags in the AR platform. Per the web site: Gen 3 has "Wide compatibility with most 'STANAG 4179' platforms such as the AR15, M4, M16, SCAR MK16/16S, HK416, MR556, M27 IAR, IWI Tavor, and others."

I've heard a few tales about the Gen 3 mags needing some massaging to fit/function in some ARs. I'd personally stick with the Gen 2s in ARs unless you've got a jones for NATO compatability.