Here is part of an excellent article by Christopher Ingraham (pictured) in The Washington Post:
It's tough to know exactly how many guns we have in the United States. Most estimates of the number of guns in the U.S. use federal tallies of the firearms manufactured, imported and exported by U.S. gunmakers. A 2012 Congressional Research Service report published exactly one month before the Sandy Hook school shooting put the number of civilian firearms at 242 million in 1996, 259 million in 2000, and 310 million as of 2009. If that 310 million number is correct, it means that the first year of Barack Obama's presidency was an inflection point: It marked the first time that the number of firearms in circulation surpassed the total U.S. population. Data on gun manufacturing from the Bureau of Alcohol, Tobacco, Firearms and Explosives now goes through 2013. If we were to update the CRS numbers with the most recent data, we'd get a chart that looks something like this: