I ordered 2 from numrich a while back for mine. They are metal, and aside from the finish seem to be identical to the metal factory ones. The factory ones were a blued finish, and the ones I got from numrich seem to be painted. I used to have 2 factory ones, one got lost years ago, the other decided to hide itself at some point during my move to ft Campbell. I used the factory metal ones a lot in the past with no major issues, I haven't used the replacement ones much yet but for at least a couple hundred rounds they have worked at least as good as my old ones. I have a friend who had one of the plastic ones, and it was fairly problematic.
I hated to spend as much as I did for the two replacement mags, but since my viper was the first rifle I saved my own money for I would never get rid of it, and it was pretty much useless with no mags.