[InclinedPlane]

Things that can be picked up by metal detectors easily: bullets, ammunition casings, gun barrels, gun slides, recoil springs, magazine springs.

Polymer cased ammunition exists but is very rare and not general purpose (e.g. flechette rounds), but even those rounds would show up quite easily on a magnetometer based metal detector due to the steel content in the flechettes. The idea of a "plastic gun" is a myth that has been advanced due to the extreme ignorance of the media inducing a moral panic in the 1980s after the introduction of the Glock 17, which merely had a plastic frame and still contained over a pound of metal in it.