What follows here isn't a policy agenda. It's simply a set of facts — many of which complicate a search for easy answers — that should inform the discussion that we desperately need to have.
(Links to pertinent documents, articles, etc., are at the end of each listing.)
1. Shooting sprees are not rare in the United States.
Mother Jones has tracked and mapped every shooting spree in the last three decades. "Since 1982, there have been at least 61 mass murders carried out with firearms across the country, with the killings unfolding in 30 states from Massachusetts to Hawaii," they found. And in most cases, the killers had obtained their weapons legally.
2. Eleven of the 20 worst mass shootings in the last 50 years took place in the United States. In second place is Finland, with two entries.
In July, Time posted the full list (via Associated Press): newsfeed.time.com/2012/07/20/the-worst-mass-shootings-of-the-past-50-years/here.
3. Lots of guns don't necessarily mean lots of shootings, as you can see in Israel and Switzerland.
As David Lamp writes at Cato, "In Israel and Switzerland, for example, a license to possess guns is available on demand to every law-abiding adult, and guns are easily obtainable in both nations. Both countries also allow widespread carrying of concealed firearms, and yet, admits Dr. Arthur Kellerman, one of the foremost medical advocates of gun control, Switzerland and Israel 'have rates of homicide that are low despite rates of home firearm ownership that are at least as high as those in the United States.'"
4. Of the 11 deadliest shootings in the United States, five have happened from 2007 onward.
That doesn't include the Newtown, Conn., shooting. The Associated Press put the early reported death toll at 27, which would make it the second-deadliest mass shooting in U.S. history.