Peter_I,
I disagree that nothing revolutionary is in 3D printing guns. A layman in terms of firearms like me can simply push a button (assuming the model and the printer are in decent quality) and obtain a firearm. It's a revolutionary producing method like all 3D printing methods to the point that individuals may produce products of their likes (maybe modify them) instead of purchasing the products from vendors. Now they only need raw materials. Business model will follow this if enough demands are there.
Next up is legal issues but I think Simpson_Jr brought it up already. It eventually is up to the people in a country to decide whether it is legal to produce certain things in a personal workshop. If anyone doesn't agree, read about whether it was legal to print materials (on paper) or own a print press in your own country say 100 to 200 years ago. Now every computer user has a printer or access to one in public print shop. Whole laws need to be made to regulate printing 3D parts, sooner or later, both as a means of obtaining products and making them for resale. Maybe in a decade we will have summer garage live 3D printing parts sale instead of garage sale of used stuff.