Yes it is harder to kill with a knife than a gun. But that's ignoring that the person still
wants to kill. If a killer is determined to kill, then I don't see how he would be turned off by not easily acquiring a firearm. Mass-killing don't happen every day in the US, murders often involve private parties (for lack of a better word). Someone who is focused on slaughtering random people is someone who is pushed far over the edge, petty "no-guns" laws won't stop him.
Marijuana is illegal in many states yet even teens are acquiring them. If there is a demand for them, the underground will supply it. It would be hell in the US when guns are banned and police will have to deal with countless dangerous illegal weapons-related busts on an astronomical scale. And the fact that the US is significantly higher in population and area doesn't help at all. One of the many reasons why the "war on drugs" is failing here is because there is no possible way to physically keep an eye out for illegal distribution and sales of contraband. I'm willing to bet drug-related crimes would go way, way down when marijuana ever gets legalized (but very regulated of course.)
Funny, this was on the front page of the BBC website:
Shots fired at resturant.
If I took your mindset, I rather let my kids tour New York City than step foot in London.
You assume that there are firefights on almost every street of a city. And a firefight in every town. Fact of matter is, a high majority of policecops in the US go through their entire career without ever firing a bullet. People buy pistols legally and never use them in their lifetime.