This harm, realistically, is minimal. Those who want to buy guns would still buy them illegally without 3D printers. Guns can be cheap in the black market since they can be mass produced, and to print a gun one first has to purchase a printer, the materials and often also the blueprints. This is similarly the case for other illegal substances. The risk that things can be used for harmful purposes is not a sufficient reason, because those who want to harm themselves or others have the means to do so already. That is why the Madrid bombers were able to develop their own bombs from the internet before 3D printers had been developed: where there is a will, there will always be a way, and it is the will and not the way that it is ever useful to tackle.