Aren’t bullets made of metal?

The whole print-a-gun-out-of-plastic thing is understandably freaking people out.  But I have a question about the following:

Security checkpoints, background checks, and gun regulations will do little good if criminals can print plastic firearms at home and bring those firearms through metal detectors with no one the wiser,”

Source:This Is The World's First Entirely 3D-Printed Gun (Photos) - Forbes

Won’t bullets set off a metal detector?