Aside from the fact that you can't buy machine guns at gun shows, has a terrorist ever had a problem getting guns? Guns are all but completely outlawed in India, but that had no effect on the Mumbai Massacre. You could completely outlaw the sale of firearms in the US and go door-to-door and pull a Diane Feinstein and you still couldn't stop a terrorist attack.
Unsurprisingly, the gun-controllers/banners are set to exploit their new terrorist friend, Adam Gadahn.
A mass terrorist shooting is inevitable on US soil and there is only one way to limit the damage of such an event. We have to do what the Indian citizens couldn't do and what the Indian police didn't do: Shoot back.