Gun-rights advocates are already suggesting that they’ll go to court to challenge President Barack Obama’s newly announced executive action on background checks. But what exactly, is the challenge going to be? And will it work?

The highlight of Obama’s action is to “clarify” the law that says checks are only necessary for sales by people “engaged in the business of selling guns.” As many as 40 percent of gun sales are currently unregulated because they’re in theory not made by dealers engaged in the business.

The most likely path for the Second Amendment activists will be to argue that the administration’s clarified definition is too narrow, interfering with nonbusiness sales. Such a claim won’t be simple to get before the courts. And when it does get there, it will be difficult for it to succeed — but not impossible.

The relevant statute, 18 U.S.C. § 921(a)(21)(C), defines a dealer engaged in the business as “a person who devotes time, attention, and labor to dealing in firearms as a regular course of trade or business with the principal objective of livelihood and profit through the repetitive purchase and resale of firearms.”

The law goes on to exclude “a person who makes occasional sales, exchanges, or purchases of firearms for the enhancement of a personal collection or for a hobby, or who sells all or part of his personal collection.”

As statues go, this one isn’t very tightly drafted. On its face, it determines engagement by the seller’s “principal objective.” If you principally want to make money, you’re dealing. If you’re building your collection, or selling it off, you aren’t.

But what about someone whose avocation is also his business? Say I love guns, love having them around and showing them to my friends, and also love selling them and making a profit on a regular basis? Am I a dealer or a hobbyist? In reality, I’m both. Legally, who knows?

Under Obama’s direction, the Bureau of Alcohol, Tobacco, Firearms and Explosives has emphasized three elements in its clarification. First, the location of sales doesn’t matter. If you sell only online or at gun shows, you can still be a dealer. That seems logical enough.

Second, the ATF implies that quantity and frequency of sales matters. Here the ATF guidance is actually confusing. On the one hand, it says that “if you repetitively buy and sell firearms with the principal motive of making a profit,” you need a license, whereas “in contrast, if you only make occasional sales of firearms from your personal collection,” you don’t. This makes it sound as if how much selling you do and how often are the key issues.

Yet on the other hand, the ATF observes that “while quantity and frequency of sales are relevant indicators, courts have upheld convictions for dealing without a license when as few as two firearms were sold, or when only one or two transactions took place, when other factors were also present.” This implies that frequency and quantity aren’t the defining issues.

Third, the ATF says correctly that it’s a crime to deal without a license, and that there are civil penalties for failing to do background checks. True — and though there isn’t anything problematic about saying this, it also doesn’t achieve much, unless it is an implicit warning that criminal prosecutions are coming.

How would a challenge to all this get into court? One way would be for the government to bring criminal charges against someone for dealing without a license. Then the defendant, with help from advocates, could say that the guidelines go too far.

That wait-and-see approach won’t satisfy gun-rights advocates. In any case, a court wouldn’t have to listen to the guidelines. The ATF’s interpretation of criminal law gets no special deference from the courts. And the ATF clarification is clearly labeled as having “no regulatory effect.”

A court could, of course, apply the definition of gun dealer more restrictively than the Obama administration wants. But in the short run, that wouldn’t help sellers who want to be sure they can avoid prosecution.

Advocates can’t plausibly claim that issuing the clarification exceeds the executive branch’s authority, like they claimed for the president’s executive action on immigration. The president’s power to state what he thinks the law means is fully covered by his duty and authority to take care that the laws be faithfully executed.

Another avenue for advocates is to seek a declaratory judgment or injunction against the clarification. This approach is tricky but interesting. Ordinarily, if I’m afraid a criminal law applies to me, I can’t challenge its application in advance. I need to wait until I’m actually charged.

An executive action that infringes on my basic constitutional rights is different. I can sometimes challenge such action as chilling my exercise of my rights, and seek an injunction.

Gun advocates might try this avenue, asserting that the clarification chills their Second Amendment right to bear arms. This claim wouldn’t be an easy one to make. The ban is only on selling without a license, not on ownership, and there’s no constitutional right to sell guns, just to own them. What’s more, the government has the constitutional authority to regulate all gun sales that affect interstate commerce, not only those by gun dealers. It’s just that Congress hasn’t exercised its power to regulate hobbyist sales.

That leaves advocates to say that hobbyists’ Second Amendment rights to own guns are derivative of their right to sell them — and that the government’s overly aggressive definition of dealership chills their ability to own the guns. This is hard argument to make.

Nevertheless, I’m not sure it’s any worse than the arguments against the Affordable Care Act that were adopted by some federal courts before being rejected by the U.S. Supreme Court. If recent history is a guide, some district court might be tempted to enjoin the Obama administration from acting on its clarification. And if that happens, it could take years for the clarification to have any impact.

Noah Feldman, a Bloomberg View columnist, is a professor of constitutional and international law at Harvard.