Black powder firearms are about as deadly as a high-power airgun. Not something you'd just shrug off, but not a modern high-velocity round, either. Also, you can't use it in cartridges without a percussion cap. The best you could do with a printer and homemade powder would be a flintlock pepperbox or a primitive revolver, with all efficiency that this implies and probably much less safety than a proper, metal gun. 3D printing is overrated, the only reason for someone to print a gun like that would be to make a political statement. Buying an actual flintlock would, in most cases, be a more viable option (even in Europe you can get them without a license, if you have the money). Or going with an airgun, some of which can be very lethal if used with right ammo and are usually not overtly regulated as well. I investigated black powder guns as personal defense weapons, and the conclusion is "airguns and gas projectors surpass them in everything but coolness".
I'm in favor of a modest increase in gun control (and standardizing it across the board), but IMO, a way to avoid school shootings would be to arm the teachers and require them to carry a gun on their person, in a secured holster. It's far easier to walk in and open fire than it is to walk up to someone and take their weapon, then start shooting. Especially that if the rest of the class realizes what the would-be shooter is up to and mobs him. Quite frankly, I don't think most shooters would have the balls to even try that. Most of this scum would rather commit suicide than pick a fight they can actually lose (as it's often seen when their victims do fight back).
A gun in possession of a trained, responsible person is not dangerous. Gun control should be designed to keep weapons out of hands of those who can't be trusted with them. Indeed, having more trained, responsible people carrying guns around increases security, as anyone dumb enough to try anything would be quickly gunned down. Restricting access to the guns usually results in people who obtain weapons from illicit sources (such as organized crime) going armed, leaving everyone with little means to fight them. This does mean the typical European mugger carries a knife or a club, but this is of little comfort if you're smaller than him and didn't take close combat classes.