Perhaps if it was literally as simple as downloading a model and pressing print, then in 20 minutes you had a fully working automatic rifle this would be an issue, but that technology simply doesn't exist today.
In reality if your goal is to acquire a weapon which can do lethal harm to someone you just wouldn't print a gun. Even if you wanted to kill multiple people in a place like the UK where guns are illegal you still wouldn't print a gun because you'd probably be better off just getting knife than printing a crappy gun and trying to source an effective propellant, etc.
ftfy