The Washington Post reports that the four guns used in the San Bernardino massacre "were all purchased legally from federally licensed firearms dealers," which means the buyers passed background checks. As usual, in other words, President Obama's knee-jerk response to mass shootings—"universal background checks"—makes no sense.

Federal officials say Syed Rizwan Farook, one of the massacre's perpetrators, bought the two 9mm pistols used in the attack, a Springfield and a Llama, from gun shops in San Diego and Corona. That means he passed background checks, which indicates he did not have a disqualifying criminal or psychiatric record. An acquaintance of Farook's bought the two AR-15-style rifles used in the attack, a DPMS A-15 and a Smith & Wesson M&P15, also at gun shops in San Diego and Corona.

The fact that the rifles were legally purchased in California exposes the fatuousness of another gun policy that Obama favors: a federal ban on so-called assault weapons. California has one of the country's strictest "assault weapon" bans, but somehow it did not forbid the sale of rifles that have been widely described in the press as "assault weapons" (or even "assault rifles," which falsely suggests they were capable of automatic fire). How is that possible?

The DPMS A-15 and Smith & Wesson M&P15 both come in "California legal" versions, which means they have "bullet buttons" that require the insertion of a loose round (or some other tool) to detach the magazine. With that feature, the magazine is not considered "detachable," which is part of the state's "assault weapon" definition.

Gun controllers tend to view bullet buttons as a sneaky end run around California's "assault weapon" ban. Huffington Post reporter Daniel Marans calls the bullet-button option a "technical loophole." Josh Sugarmann, executive director of the Violence Policy Center (VPC), complains that gun manufacturers are "cynically exploiting an inadvertent limitation" of the law. But bullet buttons are explicitly allowed by California Department of Justice regulations, which say "'detachable magazine' means any ammunition feeding device that can be removed readily from the firearm with neither disassembly of the firearm action nor use of a tool being required." The DOJ adds that "a bullet or ammunition cartridge is considered a tool."

Since rifles with bullet buttons do not have what California considers detachable magazines, they can include military-style features that would otherwise be forbidden, such as folding stocks, pistol grips, or flash suppressors. "Assault weapon" is an arbitrary, legally defined category, so the fact that California does not consider these rifles to be "assault weapons" means they aren't "assault weapons." It makes no sense to complain that California's "assault weapon" ban misses some "assault weapons," which are whatever legislators say they are. Nor does it make sense to complain about design changes, such as bullet buttons, aimed at complying with the law. Gun manufacturers that produce "California legal" guns are doing precisely what the state has told them to do.

Sugarmann tells the Post, "The gun industry is expert at marketing military-bred weapons with a wink and a nod, and they're constantly working to skirt the law, as we've seen in California." That complaint is pretty rich coming from Sugarmann, who in the 1988 VPC report "Assault Weapons and Accessories in America" argued that "the weapons' menacing looks, coupled with the public's confusion over fully automatic machine guns versus semi-automatic assault weapons—anything that looks like a machine gun is assumed to be a machine gun—can only increase the chance of public support for restrictions on these weapons." Targeting so-called assault weapons was always about emphasizing "menacing looks" in the hope of confusing the public, and to this day leading supporters of laws like California's misrepresent the firearms they want to ban. That includes Obama, who describes these semiautomatic rifles as "fully automatic…weapons of war," and Hillary Clinton, who conflates them with "machine guns."