The reality is that that thought scares the living shit out of everyone and its become culturally accepted that there is a line in the sand on what types of weapons should be available to people. Perhaps the line for the US needs to be redrawn.
As has been shown in other countries (specifically, Australia where I live) when you reduce the availability of weapons that are designed purely for mass killing (our buyback scheme went pretty smoothly in 1996), the ability of people to do mass killing goes down. We have approximately 1/3 the number of firearm related deaths[1] (both accidental and not) than the US has of just homicides alone per 100k people[2].
Thats not to say guns aren't still available. They are. That's not to say people still don't get killed with guns, because they do. But the numbers just dont lie.
[1] http://www.gunpolicy.org/firearms/region/australia [2] http://www.cdc.gov/nchs/fastats/homicide.htm