This is all fanciful thinking. Even if guns were more difficult to get, he could lay hands on a pipe bomb, Molotov cocktail, or just drive a truck into the crowd. Guns could be flat-out illegal, and I think a determined wannabe mass murderer could still find one.

That's true enough, but its not as straightforward as that. If the intention is merely mass murder, that argument holds. But there are lots of other circumstances where the emotional/psychological position of power over the each individual victim seems to have been important and those weapons don't do that. I'd put Columbine in that group, for example.