Just how many gun purchases don't require federal background checks, and how does that happen?

Forty percent of all firearms purchased in the United States are sold without background checks because the guns aren't bought from a federally licensed firearms dealer, Nichols said.

Rather, those weapons are bought at gun shows, on street corners, over the Internet or from friends or neighbors, Nichols said.

These are the so-called loopholes in the current federal background check system.

The NRA disputes that characterization about the "gun show loophole" because federally licensed firearms dealers participate at gatherings and, of course, conduct background checks.

"Most of the guns that are purchased at a gun show are purchased from federal firearms-licensed holders," Keene said.

He challenged the 40% figure for gun sales without background checks -- particularly at gun shows.

"We don't know what (is the) percentage at gun shows. It may be 10%," Keene said. "It's not such a loophole at gun shows. But it's like if you sell me your shotgun, that's a private transaction. Just as if I sell you a car, I don't have a dealer's license."

Ten states and the District of Columbia have their own laws requiring background checks for any firearm sold at a gun show, Nichols said.

Six more states require background checks for gun-show sales of handguns -- but not for rifles or shotguns, Nichols said.

In total, 16 states and the District of Columbia require background checks on handguns sold at gun shows, Nichols said.

These states that close loopholes, however, provide exemptions for gun transfers between immediate family members and between licensed dealers, Nichols said.

Are background checks effective?

From the time when the gun control measures of the Brady Act were enacted on March 1, 1994, through the end of 2008, the federal government processed more than 97 million applications for gun transfers or permits, the Justice Department says.

Almost 1.8 million applications were denied, the agency said.

On this matter, both sides are in agreement.

Said Keene: "Background checks are generally a good thing."

Added Nichols: "Background checks have a huge deterrent effect. People who are ineligible to buy a gun are unlikely to try if they know they are going to be subjected to a background check."