You make it sound simple, but where would that line be drawn? Be specific.

If you think it would ban semi-automatic rifles, or handguns (both of which were found in the attacker's hotel room), then you're overlooking their numerous civilian uses: hunting, home defence, pest control, sport, etc.

If you were just thinking of banning machine guns, then you're in luck: fully automatic weapons are already tightly restricted by US federal law. But it's not clear at this point that the attacker actually used any such weapons. He may instead have made modifications or used accessories to enable quicker firing, but that has not been confirmed. If he did convert a semi-auto weapon to fully automatic fire, that would've been illegal too.

What I'm saying is, there's no easy fix here. Either the line was already drawn (and didn't stop this massacre), or it would have be drawn in a place that would turn legitimate civilian users into criminals.