American gun control supporters are citing the firearm restrictions that New Zealand's government plans to impose in response to last week's mass shootings at two mosques in Christchurch as an example that should be emulated by American politicians. But the broad gun and magazine bans that legislators expect to enact by April 11 would never pass constitutional muster in the United States. If we can learn anything from Prime Minister Jacinda Ardern's reaction to the attacks, it has less to do with the merits of her policies than with the slippery language she used in announcing them.

"The guns used in these terrorist attacks had important distinguishing features," Ardern said at a press conference in Wellington today. "First, big capacity, and also their delivery. They had the power to shoot continuously, but they also had large capacity magazines."

Contrary to that description, the guns used by the perpetrator of the mosque attacks, which killed 50 people, did not "shoot continuously." They were semi-automatic rifles, meaning they fired once per trigger pull. And while Ardern referred to "important distinguishing features," the only one she mentioned (twice) was "big capacity," which is a characteristic of the magazine rather than the gun itself.

Ardern does plan to ban "high-capacity magazines," meaning those holding more than five rounds. There will be an exception for magazines holding up to 10 rounds of .22-caliber or smaller rimfire ammunition.

Ardern also intends to "ban all military-style semi-automatic weapons" (MSSAs), which under current law include semi-automatic rifles that have pistol grips, folding or telescoping stocks, bayonet lugs, flash suppressors, internal magazines holding more than seven rounds, or detachable magazines that have "the appearance of holding more than 10 cartridges" (15 for .22-caliber rimfire ammunition). MSSAs already require a special license. Ardern wants to make them entirely illegal, and that includes firearms currently owned by license holders, who will be required to surrender them. They are supposed to receive compensation, but this "buyback" won't be optional.

In addition to banning the guns already classified as MSSAs, New Zealand is expanding that category to include all semi-automatic firearms capable of accepting magazines that hold more than five rounds (or 10 in the case of .22-caliber or smaller rimfire ammunition). That's a big deal, because the prohibited guns will include many models of handguns and rifles that accept detachable magazines but were not heretofore considered MSSAs. The Christchurch shooter reportedly bought two semi-automatic rifles with a standard gun license, then "modified" them to fire more than seven rounds, meaning he switched to bigger magazines. Now Ardern wants to ban not only larger magazines but also the guns capable of accepting them.

"We will also ban all assault rifles," Ardern said. If that category is supposed to be distinct from MSSAs, it's not clear what Ardern means. Traditionally, "assault rifles" were military guns capable of automatic fire, but those are already deemed "restricted weapons" in New Zealand, meaning they can be legally owned only by special license holders such as collectors or movie producers, who are not allowed to fire them.

In the United States, politicians, journalists, and gun control activists who talk about "assault rifles" often treat the term as interchangeable with "assault weapons," an arbitrary category defined by law. While the criteria vary from one jurisdiction to another, "assault weapons" in the United States are similar to New Zealand's original definition of MSSAs in the sense that the category includes guns with "military-style" features, such as folding stocks, bayonet lugs, and flash suppressors, that do not make the weapons any more deadly in the hands of a mass shooter.

By contrast, New Zealand's new definition of military-style semi-automatics, which will now be not just restricted but banned, hinges on a functionally significant distinction: the ability to accept detachable magazines. At the same time, that criterion is so broad that it renders the term military-style meaningless and sweeps in a wide range of firearms used for lawful purposes. In the United States—which, unlike New Zealand, has a constitution that guarantees the right to armed self-defense—such a sweeping ban would be inconsistent with Supreme Court rulings rejecting bans on semi-automatic handguns. Yet it is more logical than American-style "assault weapon" bans, which focus on looks rather than lethality.

The "military-style semi-automatic weapons" New Zealand plans to ban (which may or may not be synonymous with the "assault rifles" Ardern "also" wants to ban) include many guns that never fell into that category before and do not necessarily have anything to do with the military. It is therefore more than a little confusing to continue using the same term for them, especially for Americans who imagine that the category is equivalent to what U.S. politicians have in mind when they refer to "assault weapons."

Since the rationale for such laws is that they make it harder to obtain guns that are especially suitable for mass murder, the details matter. "Assault weapon" bans tend to draw distinctions that make no sense in light of that goal. New Zealand's government is implicitly acknowledging that problem by focusing on function instead of appearance. But that also means imposing a much bigger burden on law-abiding gun owners, who now will be required to give up detachable magazines and the guns that accept them, which are surely more widely useful than bayonet lugs or flash suppressors. In that tradeoff, there is a lesson that American gun controllers should take to heart.