And plastic isn't detected by a metal detector (there's a movie plot about this! [+]).
Usually these 'homemade' guns buy a lot of professionally made gun parts such as the rifle barrel and just make their own lower receiver, which isn't exposed to the explosive high pressure part of a gun.
Disclaimer: I’m familiar with this only because I have to check my firearms when I’m flying.
Everyone there had to wear a white shirt and pants. There was only one entrance and there was a metal detector right in front of it. But what all the "cool" kids were doing was seeing how big of a knife they could sneak through it. And there was a couple of 3 inchers that got through. It mattered were the knife was and how you held your hands as you went through, but can't remember the details (this was back in 2001).