A growing number of incidents have highlighted the dangers of 3D-printed ‘ghost guns’, untraceable firearms that can be assembled at home with the help of a 3D printer and a set of blueprints. Since the first design appeared in 2013, 3D-printed gun technology has advanced rapidly. Some models can now fire hundreds or thousands of rounds without their plastic components failing. Although these weapons are illegal in many jurisdictions, designs, parts, and blueprints continue to spread on social media. We explore the growing popularity of 3D-printed weapons online.