A small percentage of people are denied guns while others don't follow through with the gun purchase despite having a background check, said Tom Smith, director of the General Social Survey at the University of Chicago's National Opinion Research Center (NORC), a social science research group established in 1941. Also, background checks only include purchases through licensed dealers. Some can legally purchase guns in private sales without checks. Illegally purchased or transferred guns are, of course, another problem.
Smith said it could be difficult for the firearms industry to aggregate their sales or ownership data, and not just for competitive reasons. Among the other possibilities are there are increasing numbers of small manufacturers and imports of guns into the U.S.
"I'm not sure if people in the industry even know what the total numbers are with each company keeping its own private records," Smith said.
One piece of information researchers found is that the percentage of households with a gun has declined from 1973 to 2011, according to Smith.
In 2010, about 32.3 percent of American households reported having any guns in the home, after hitting a peak in 1977, when 54 percent of households reported having any guns.
As Smith points out, however, "If a household has 20 handguns or one, it's still one household."
Josh Sugarmann, executive director of the Violence Policy Center, said that even if gun ownership is declining, the gun industry's "bright spots" in sales are assault weapons and what the gun industry calls self-protection handguns marketed as able to be concealed.
"Grandfather's rifle or the traditional market is fading," Sugarmann said.
Sugarmann said it is possible that the number of guns per owner in the U.S. is increasing.
"Certain guns develop a cache and they are sold for different reasons," he said.
A key element of the gun industry's sales strategy is to market assault rifles to law enforcement and the army, "which is legitimate use, and then they use a halo effect for creditability to market to the civilian marketplace," Sugarmann said.
The common thread in mass shootings in the U.S. is "very few pistols sold today have magazines with less than 10 rounds," he said.