The reason is simple: 20 times more people are killed by handguns than rifles -- including assault rifles -- every year.

In the U.S. in 2011, there were 6,220 murders committed with handguns and 323 with rifles, according to the FBI. The government doesn't differentiate between regular and "assault" rifles (the definition of which is contested in any case), meaning the share of killings with assault rifles, however you define them, is surely an even smaller percentage. That means that even the most effective possible assault-weapons ban -- one that got every one of the weapons targeted off the streets -- would only affect a small portion of gun deaths. A reform that keeps handguns out of the wrong hands, by contrast, could have a big impact, even if it's only marginally effective.

How much impact is hard to say, in part because NRA-backed regulations have made it very hard to collect data on gun purchases. For example, all background-check records are required to be destroyed within 24 hours, making it impossible for law enforcement and researchers alike to track attempted purchases.

The way the system now works is this: Would-be gun buyers must undergo a background check if they purchase from a licensed firearm dealer, defined very loosely in the law as someone who makes a living selling guns. Collectors, hobbyists, and many other sellers are exempted; at gun shows, exempted sellers will set up tables with signs reading "No Background Check!" or "No Questions Asked!" There's even a "Craigslist for guns," the online marketplace Armslist.com.

A commonly cited statistic places the portion of guns purchased under the "gun-show loophole" at 40 percent, but that's based on a flawed survey conducted nearly two decades ago, when background checks had just started to be required, as the Washington Post 's Glenn Kessler has noted . Gun-control advocates generally think the percentage is lower now that the 1993 Brady Bill has been fully implemented. A newer but still inconclusive survey put the portion of guns used in crimes that were purchased from licensed dealers at 11 percent. Washington Post

Convicted felons, people who have been legally judged to be mentally ill, and those dishonorably discharged from the military are among the categories of people banned from buying guns. Advocates say the existing background check process isn't onerous -- it takes an average of seven minutes to complete, using either an online database or phone hotline. Is it effective? The denial rate last year was just 1.5 percent and has declined over the years, presumably as more illicit buyers are diverted to the unregulated part of the market. But 1.5 percent of 16 million -- the number of background checks conducted last year -- is still a fair amount of people who shouldn't have guns trying and failing to get them. And if more sellers had to conduct checks, advocates say, more guns would be kept out of the wrong hands.