It is difficult to know how many private guns are in the hands of Americans. With no central database, the government and independent researchers are left to make their own tallies.

The man accused of killing 11 and wounding six in a Pittsburgh synagogue carried four guns in the Oct. 27 attack, an AR-15 semi-assault style rifle and three handguns.

One estimate of U.S. gun ownership is calculated using data from the Bureau of Alcohol, Tobacco, Firearms and Explosives, or ATF. This figure tallies the number of firearms produced by American manufacturers, adding the number of firearms shipped from other countries by American importers, and subtracting the number of firearms exported internationally. By this count, the U.S. has more guns in nonmilitary hands than residents.

But this estimate comes with several caveats. Annual manufacturing and import numbers don’t necessarily equate to annual sales. The ATF definition of a firearm includes specific parts regulated by the 1934 National Firearms Act, such as silencers, inflating the count. And it includes weapons manufactured for law enforcement agencies, though not the military.

Tallying up the annual ATF numbers to arrive at a cumulative total of guns in America also fails to account for firearms which may have been destroyed, confiscated, permanently lost or otherwise taken out of commission.