If a wizard suddenly made it impossible for guns to exist in America; they could not pass across any border, the ones inside the country simply turned into nothingness, do you think the rates of assault and murder would instantly go down? Or do you think they'd stay the same, just with different methods?
Say, for example, that ten people are killed per year; five by gun, two by knife, two by baseball bat, and one by strangulation. On Dec 31st, the wizard casts his spell.
What do you think the stats will be one year later? Two by knife, two by baseball bat, and one by strangulation? I don't. I think they'd turn to four by knife, four by baseball bat, and two by strangulation.
Guns are a symptom, not a cause. Unless the root causes in American society are addressed, people will continue to die. The manner of their murder shouldn't even be a point of discussion.