Hard to believe Adams is clueless about plastic guns [such as the Liberator (edit)]. Gun detectors are pure graft unless he is planning a full TSA strip search to use the subway.
Even more robust DIY guns like this use metal components for only the breech, and that's a very small component that could easily be confused for any number of other things.
Breach only makes a lot of sense. That would indeed be hard to detect, and would likely solve the above.
Apparently the 380 cartridge doesn’t explode upon firing