The next shoe to drop on gun control may come by mid-January, when President Barack Obama is expected to issue an executive order requiring everyone "in the business" of selling firearms to perform background checks.

Wait a second, you might be saying. Doesn't federal law already oblige gun retailers to do computerized criminal checks via the Federal Bureau of Investigation's data base? Yes and no.

Yes, when it comes to federally licensed dealers. But no, when you're talking about people who lack federal licenses and sell guns from their personal collections.

The problem is that an awful lot of firearms are sold in the latter fashion by individuals who aren't technically gun retailers but who sell weapons at weekend gun shows or from their homes. Forthcoming research by the Harvard School of Public Health estimates that 40 percent of all gun transfers occur without background checks (that's the so-called gun show loophole). Presumably the background-check gap permits some criminals and mentally disabled people to buy guns who otherwise might be stopped.