The Newtown shooter must have had the longest arms in the history of humans, since the rifle was found in his car. As for semi-automatic pistols, they are also my weapon of choice for self-defense against nut jobs -- mine is never out of my reach. Rifles are expensive and very unhandy.
A lot of it depends on venue and what is available. The largest mass murder in US history happened in Michigan in 1927 -- the guy blew up a school with dynamite and killed 38. Ragheads, on the other hand, use car bombs and IEDs. Angry women have been known to just burn the place down with everyone in it.
If they were really serious about stopping this madness, they would be looking for ways to treat and institutionalize the kooks and nut jobs.