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Main differences are aesthetics and a few geometry changes to offer better functionality in weapons such as the 416 and M27 IAR, I'm guessing specs similar to the EMAG. Maybe Magpul will swing by and let us know.
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Yes the M3s addressed compatibility issues with non AR15/M16/M4 weapon systems such as the IAR (HK416), SA80A2, FN SCAR and IMI Tavor. The top half geometry is very similar to the EMag used as standard issue by all UK forces in Afghanistan for 3 years.
The M3 also offered increased performance in extreme environments (for example, the M3 is temperature rated down to -60 where as M3 is rated down to -30)
Other improvements included grip texture on the front and rear of the magazine, low profile floorplate, improved magazine retention area, improved feeding geometry for adverse conditions and paint-pen marking matrix (the dots on the side allowing paint pen info to be recessed/protected from being scratched off)
All this being said the Gen M2s uses the new M3 material (not the M3's advanced processing) and the M2 has a solid 5 years of being the largest combat fielded polymer magazine (for the M4/M16) in US Military history. In Iraq and Afghanistan it out numbered all other polymer magazines, combined, so at the prices of M2s currently you really cannot go wrong with them either.
For those who want to see the M3 PMag running through some extreme testing here are a series of test videos we put together using high speed video (6000-10000 fps) with test firing validation of function using a factory full auto HK 416.