Some gun law experts say the Second Amendment prevents President Trump from implementing a ban on young adults buying AR-15-style rifles.

Trump endorsed limiting sales of the semi-automatic guns to adults 21 and older after a 19-year-old murdered 17 people at a school last week in Florida. Although legal opinion is divided, several experts believe federal judges would strike down a purchase ban affecting 18-to-20-year-olds.

The one-sentence Second Amendment doesn’t mention age, and views vary on its precise meaning. But over the past decade, courts have taken an expansive view, with the Supreme Court recognizing a right to own handguns and many lower courts easing concealed carry rules.

Some federal courts have dealt with a different firearm age limit, upholding the 21-year-old age requirement for handgun sales by licensed dealers. That rule passed Congress in the Gun Control Act of 1968, long before the AR-15 became one of the nation's most popular guns, and doesn't ban young-adult handgun possession.

A fiery 2013 dissent from Fifth Circuit Court of Appeals Judge Edith Jones suggests a divided judiciary on the handgun sales restriction.

In her dissent, Jones wrote: “Congress has seriously interfered with this age group's constitutional rights because of a class-based determination that applies to, at best, a tiny percentage of the lawbreakers among the class. Of course, the lawbreakers obtain handguns, but the law-abiding young adults are prevented from doing so, which adds an unusual and perverse twist to the constitutional analysis.”

The Supreme Court declined to review the Fifth Circuit decision, leaving the age-limit question unsettled. But several attorneys involved with gun-rights litigation say they see its days as numbered and believe the justices would also knock down a new AR-15 age limit.

Michael Connelly, executive director of the U.S. Justice Foundation, a public interest law firm that supports gun-rights advocates, said, “I think an effort to ban people between 18 and 21 from purchasing AR-15s would be vulnerable to a Second Amendment challenge, particularly in light of the current makeup of the Supreme Court."

“The Second Amendment does not provide an age limit,” Connelly said. “It simply states that the right to keep and bear arms shall not be infringed upon. It must be remembered that at the time preceding the revolution most colonies required men 16 and older to be in the militia and provide their own firearms. Thus there was no age limit on their ownership put in the Constitution.”

Connelly said that, by contrast, “there was an age limit for voting and it had to be changed by the passage of the 26th Amendment,” and that “therefore, unless Congress and three fourths of the states choose to amend the Constitution and place an age limit on the Second Amendment, the simple passage of legislation to do so would probably not pass constitutional muster.”

Connelly noted the military pulls heavily from the 18-to-20-year-old age group and that “saying that this same group is too immature to own certain firearms privately would be a tough argument to make.”

Attorney Stephen Halbrook, a senior fellow at the Independent Institute who has written briefs for and argued gun-rights cases before the Supreme Court, also believes a higher age limit would be unconstitutional.

“The proposed ban on federally licensed dealers selling rifles, including the AR-15, to persons in the 18-to-20 age group would, in my view, violate the Second Amendment,” Halbrook said.

“At the time of the founding, 18 was the age for membership in the militia under the federal Militia Acts of 1792 and the state militia laws. Each militiaman was required to provide his own musket or rifle. Today, 18-year-olds can join the military and can vote. They are emancipated citizens,” he said.

“I predict that there will be significant pushback to a rifle sale ban on the 18-20 age group, as that has never been illegal in U.S. history,” Halbrook added. “I’m unaware that there was ever pushback on the handgun sales limitation to 21-year-olds, as that had been the law in a number of states before [1968].”

Attorney Jeffrey Teichert said that in addition to legal claims of Second Amendment protection for young adults, people could challenge a ban on equal protection grounds, arguing wrongful discrimination based on age.

Teichert wrote an amicus brief in the 2008 case District of Columbia v. Heller, in which justices declared a personal right to possess firearms in “common use” for lawful purposes, and said he believes the handgun age limit may have survived for decades because it’s seen as a low priority. He believes it will eventually be overturned, however, following Heller.

“Being involved in briefing the Heller case, we were cautious not to move too quickly or give the Supreme Court something they weren’t able to do. I guess there are practical considerations as well, people want to get half a loaf rather than none,” he said.

Teichert, offering an example of strategic decisions not to sue, said it’s possible to argue that the Second Amendment protects the right to bear arms for felons and the mentally ill, but that nobody would want to risk an unfavorable ruling pressing such a radical position.

Teichert sees a “distinct possibility” that some gun-rights groups would decide that denying young adults is an acceptable trade-off to take pressure off ownership rights for older adults, though he said the idea bothers him and that it would be "ludicrous" to consider age restrictions on other constitutional rights, such as a 35-year age restriction to write articles for a newspaper.

Currently, just two states — Illinois and Hawaii — have a 21-year age limit to buy an AR-15, according to the Giffords Law Center to Prevent Gun Violence. Most states default to the federal 18-year age limit.

Among some gun-rights advocates, there's doubt that courts would rule in favor of a young adult's right to own guns.

Attorney Dave Hardy, a Second Amendment expert cited in majority and concurring opinions in the 2010 McDonald v. City of Chicago case, in which the Supreme Court found the Second Amendment overrides state laws, said he isn’t sure how the courts would rule.

“I think it might hinge upon things we don't know. For example, is there any evidence that 18-to-21-year-olds more often use rifles or rifles of a certain type in crime than person 21-plus? Is it a significant difference? I can't say because I've seen no data of that type,” Hardy said.

“Probability of a challenge: I'd say likely, at least somewhere above 50-50,” Hardy said.

George Mason University law professor Joyce Malcolm, meanwhile, said she believes that the age restriction would be unwise, but likely allowable.

“I think it would be constitutional to ban adults 18-21 from buying AR-15s but not make much sense,” she said, adding that “any effort to prohibit young adults 18 and over from exercising a constitutional right would face serious challenge. If they can't handle a common long gun why do they have the right to vote? Serving in the military they handle far more dangerous weapons. The effort seems to be to simply chip away at the right, rather than take seriously the need for school security and to deflect attention from the lapses of the police and FBI.”

Advocates of gun control say a higher AR-15 age limit would be within the realm of reasonable restrictions courts have allowed and that Congress could go even further.

“The Second Amendment applies to all adults, but it does not answer the question of what regulations are allowed,” said University of California, Berkeley, law dean Erwin Chemerinsky.

“The government can — and should — ban the AR-15,” Chemerinsky said. “That would not violate the Constitution. As a lesser measure, it could raise the age of ownership.”

“Raising the gun age is likely constitutional,” agreed University of California, Los Angeles, law professor Adam Winkler, whose research has been cited in several major gun-rights rulings.

“The Second Amendment is limited to law-abiding adults. And while the Supreme Court has not said what age limits are appropriate for the Second Amendment, they are unlikely to say that the Second Amendment is offended by banning 18-to-21-year-olds from purchasing rifles,” Winkler said.

Winkler conceded it’s not possible to limit First Amendment rights for young adults, but said lawmakers can limit the Constitution’s gun ownership grant because “the rights are just different.”

Trump frequently makes off-the-cuff remarks, but repeatedly stated his support for a higher age limit in the aftermath of the Florida school massacre, which came after AR-15s were used in many other mass shootings too.

“We're going to work on getting the age up to 21 instead of 18,” Trump said during a Thursday event at the White House with local and state officials.

“I mean, we're talking about rules and regulations for purchasing. We're talking about changing an age, from 18 to 21,” he said. “I mean, so they buy a revolver — a handgun — they buy at the age of 21. And yet, these other weapons that we talk about that some people don't like — they're allowed to buy them at 18. So how does that make sense? How does that make sense? So I say that it should all be at 21.”

Trump separately told reporters Thursday that he believed the National Rifle Association would back the age-limit increase.

"I don't think I'll be going up against them,” he said. “I really think the NRA wants to do what's right. I mean, they're very close to me. I'm very close to them. They're very, very great people. They love this country. They're patriots. The NRA wants to do the right thing. I've spoken to them often in the last two days, and they want to do the right thing. They're going to do the right thing, I have no doubt in my mind."

It’s unclear if Trump backs a ban on AR-15 possession by young adults or merely a ban on federally licensed dealers selling the guns, as is the case with handguns. Halbrook, one of the pro-gun rights attorneys, said a more limited restriction may, ironically, have a worse public impact, by sending young buyers in search of private sellers who don’t have to conduct background checks.