Guns are an interesting example. Printing a 3D gun is somewhat viable but a 3D printer is not a dispenser where you can just press a button and a perfectly crafted glock pops out for your 5 year old. 1) You have to have strong enough materials for the casing 2) The inner surface of your barrel better be smooth (something even high precision 3d printers are relatively bad at) 3) If 1 or 2 is in any way flawed its going to be your own hand thats blown off - better get it right.
3D printing is going to have to compete with a global distribution system that is already extremely efficient. Take for example coffee cups - great example of something to print but I'd probably never print a coffee cup. Why? Because they are damn cheap and ubiquitous. By the time I get done with downloading the model, purchasing the materials, printing the cup, preparing the surface (shining, painting, whatever) Ive spent more time and money than just hopping over to Walmart and spending a few bucks. Guns within the rings of people who want them in mass who you are mostly likely worried about are in a similar situation - they can get one down the street.
On top of it, 3D printing currently competes pretty much only with the lower tier of manufacturing... stuff made out a single material with usually at most a few parts. Whenever I see what can be done with a 3D printer its honestly not that compelling - toys, utensils, models, etc. I think 3D printing has potential but also think it will be a long time until everyone is printing everything they want.