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The mag release is metal. It appears to be blued steel but I can't say for sure. It is subjected to rather frequent wear that would wear the bluing off of it. I guess it's possible that at some time in history Kahr used a plastic magazine release but considering how this thing works, that would be very stupid and would wear out in one range session.

The spring in my CW9 appears to be stainless steel. It is the second one. The original one was also stainless AFAICT.

I suspect it's sweat causing the rusting. I don't have this problem with my Kahr but I suspect it's because of my choice of holster which does not put the mag release in contact with my skin.