I could be wrong, but I think, if you take a plastic gun apart and examine the slide and rails, you will find no plastic in the linkup between them.
Every plastic-framed gun I know of has metal inserts to form the bearing surfaces of the rails.
Between the parts, rails, slide, barrel lugs, barrel bushing and whatever link mechanism (ramp, slot, etc). It will be all metal. I could be wrong, as I said, but I can't think of any.
Lost Sheep
I wasn't thinking so much about rails. I realize all plastic guns have steel inserts for rails. I was thinking more of the tendency to flex badly when fired due to the swinging link design. In order to overcome that, you would have to have quite a bit of steel in that area.