Former Vice President Joe Biden, the leading contender for the Democratic presidential nomination, today got into a heated argument with a Detroit autoworker who challenged his support for a new federal "assault weapon" ban. Even leaving aside Biden's reference to an "AR-14" when he meant "AR-15," the conversation revealed both the illogic of his proposal and the suspicions it understandably arouses among many gun owners.

Cellphone video of the encounter shows a bearded man in a hard hat accusing Biden of "actively trying to end our Second Amendment right and take away our guns." Biden denied the charge. "You're full of shit," he said. "I support the Second Amendment. The Second Amendment, just like now, if you yelled fire, that's not free speech…I have a shotgun. I have a 12-gauge, a 20-gauge. My sons hunt….I'm not taking your gun away at all."

It is true that Biden's proposal—like the 1994 federal "assault weapon" ban, which expired in 2004—does not include confiscation of guns Americans already own. Instead he would give owners of the targeted firearms a choice: They could sell their guns to the federal government, or they could register them under the National Firearms Act (NFA), following the same procedure, including a background check and a $200 tax, that applies to machine guns. Unlike former presidential candidate Beto O'Rourke, Biden is not threatening to "take your AR-15."

But state requirements for registration of "assault weapons" have been honored mostly in the breach, and Biden's plan is likely to be even less successful now that talk of confiscation is in the air. When the government does not know who owns the guns it decides to ban, it can neither force people to register them nor seize them. It is perfectly rational for gun owners to worry that the first step will eventually lead to the second.

During the exchange in Detroit, Biden himself muddied the legal impact of his proposal. "Are you able to own a machine gun?" he asked. "No, machine guns are illegal," the autoworker replied. "That's right," Biden confirmed. "How are AR-15s legal?"

It's not actually true that "machine guns are illegal." While new production for civilian use has been banned since 1986, machine guns owned before then can be legally possessed and transferred as long as the NFA's requirements are followed. On one hand, Biden wants to treat "assault weapons" the same way machine guns are treated, which he says shows he does not favor confiscation. On the other hand, he erroneously says no civilian is legally "able to own a machine gun," which contradicts his first point.

Biden argues that machine guns "are rarely used in crimes" because of the restrictions imposed by the NFA. But even without those restrictions, "assault weapons" also are used in a very small share of gun homicides. In 2018, according to the FBI's numbers, rifles in general—only a subset of which would qualify as "assault weapons"—accounted for 4 percent of guns used in firearm homicides where the type of weapon was specified. Handguns, by contrast, accounted for 93 percent of the weapons used in those cases. A tally by Sen. Dianne Feinstein (D–Calif.), who sponsored the original "assault weapon" ban and has introduced a new, stricter version that is probably similar to what Biden favors, suggests that the firearms she considers intolerable were used in something like 0.5 percent of gun homicides from 2004 through 2011.

The argumentative autoworker raised that point with Biden, noting that handguns are much more commonly used in homicides than the firearms he wants to ban. "Why are you advocating for [a ban on] assault rifles?" he wondered. Biden did not answer.

There is a good reason for that. Biden has conceded that the 1994 "assault weapon" ban had no impact on the lethality of legal firearms, which remained "just as deadly." He says he would fix that problem, but it is hard to see how, since "assault weapons" are an arbitrarily defined category of firearms distinguished by military-style features that make little or no difference in the hands of a murderer. No amount of tinkering with the list of forbidden characteristics can ban guns that are effective in mass shootings without also banning guns that are commonly used for self-defense and other legal purposes, which would clearly violate the Second Amendment.

Biden wants us to believe that owning an AR-15 is constitutionally analogous to "falsely shouting fire in a theatre and causing a panic," which is "not free speech." But he cannot explain why. The Supreme Court has said the Second Amendment guarantees the right to own firearms "in common use" for "lawful purposes," a standard that so-called assault weapons easily satisfy, since they are among the most popular rifles sold in the United States.

Today Biden repeatedly asked his interlocutor whether anyone really needs a magazine that holds "100 rounds," which is doubling misleading. First, the issue of ammunition capacity is distinct from the definition of "assault weapon," since a gun could fall outside Feinstein's criteria and still accept a 100-round magazine. Second, Biden's proposal to ban "high-capacity magazines," assuming it is similar to Feinstein's, draws the line at 10 rounds, not 100. That rule would ban magazines commonly used for self-defense.

To show that he supports the Second Amendment, Biden noted that he owns shotguns and that "my sons hunt," which is not exactly reassuring for anyone who values the right to armed self-defense. Biden also has said that if you must keep a firearm for home defense, a shotgun is the way to go—questionable advice that has been rejected by the millions of Americans who own handguns for that purpose, a choice the Supreme Court has recognized as constitutionally protected. Feinstein seems to share Biden's affection for shotguns, hundreds of which are included in her bill's gratuitous list of specifically exempted firearms.

Since even shotguns are more commonly used in homicides than "assault weapons" are, the constitutional or public safety distinction that Biden and Feinstein have in mind is rather mysterious. If Biden wants gun owners to believe him when he says he respects the Second Amendment, he will have to do a better job of explaining which rights he thinks it protects and why.