Re: A gun is involved in every single mass shooting.
"you could eliminate legal sales or possession of guns, there is a huge black market for guns"
In the US there are so many shootings that are not done by gangs, cartels, criminals who have easy access to illegal weapons. The majority of fatal shootings is done by legally registered / acquired weapons. It's a fallacy to think that if it were much more difficult to acquire weapons legally that Mr. white-collar-worker would turn to the criminal underworld to acquire a gun.
"real issue which is not guns but evil, sick depraved people"
You're right that sick people are the root cause. However it is relatively difficult to mass-poison people, acquire explosives and build a large bomb etc, certainly compared to how easy it is in the US to buy a gun and start shooting. At least try and make it a bit hard for the nutters.