No it isn't.
There's little doubt that the fact that gun-related mortality is so high in the US compared to other western nations[1] is caused by the fact that any fool in the US can buy a gun legally, and does.
I'm well aware of the fact that limiting legal gun ownership will not eradicate this kind of mass murders. The man who killed six people in a shopping mall in Alphen aan den Rijn in the Netherlands last year did so with guns he owned with a government permit. There's also the issue of illegal gun ownership.
[1] For a comparison see this Wikipedia page. The only western European country that comes close is Switzerland, another country where guns are widely available.
regards,
Hein