One simple refutaion demonstrated millions and millions of times over:

In every month since Obama's election, enough firearms have been sold to individuals to arm the combined armies of Russia and India. Millions and millions and millions.

If guns cause violence, we should have corresponding millions and millions and millions of dead bodies on our hands, every morgue should be full and all our rivers should be running red.

But they aren't.

Leaving us but two possible conclusions:

1. Guns don't kill people.

2. All those millions and millions and millions of guns are deffective.