There's el cheapo plastic, like model cars and planes, and then there's the polymers, like modern gun frames.
Big difference.
Just make sure what the piece is made of.
Always do a test before getting carried away with using anything on a non-metal part.
Some plastics will positively melt and polymers will suffer no real harm.
Except maybe for the finish.
So, always test first.
Anyone want a thoroughly melted .22 magazine that didn't get tested before getting sprayed?