Can’t be a law-and-order president if you’re not willing to Do Something about mass shootings.

The new regulation on bump stocks is easy to defend insofar as it doesn’t limit adults’ access to any type of legal firearm. On the contrary, it makes sure that legal firearms don’t behave like illegal ones. You can think of it as closing a “technological loophole,” which is why even the NRA supports it. Taking a class of firearms entirely out of the hands of part of the adult population is dicier, though, and obviously an escalation in Trump’s gun-control thinking. Most gun-control debates reduce to slippery-slope arguments: If “assault weapons” are unsafe in the hands of adults aged 18-20, why aren’t they also unsafe in the hands of adults aged 21-25? Just three days ago, Ross Douthat proposed limiting them to adults aged 30 or older. Granted, we use the age of 21 as a legal benchmark for all sorts of things, but there’s no reason why a public safety measure would necessarily need to be pegged to that number.

If Trump gets his ban and next month a 28-year-old kills 50 people at the local college campus, what does he do then?

In private, he has indicated that he might do more, telling advisers and friends in recent days that he is determined to push for some sort of gun-control legislation, according to people familiar with the conversations.

In one such discussion, during dinner with television commentator Geraldo Rivera at Trump’s Mar-a-Lago Club in Palm Beach, the president listened with interest as Rivera suggested raising the minimum age at which a person could buy a semiautomatic weapon from 18 to 21. White House press secretary Sarah Huckabee Sanders told reporters Tuesday that the idea is “on the table for us to discuss.”

Eh, that’s Geraldo talking, not Trump, right? Well, no: Per Axios, Trump himself has mentioned raising the age limit on gun purchases to 21, saying, “We have to do something. We’ve got kids dying.” He’d have an easy argument in his defense too, and I don’t mean pointing to the fact that the Parkland shooter was under 21 when he bought his AR-15. It’s already illegal under federal law to purchase a handgun if you’re under 21; long guns, however, can be purchased by anyone over the age of 18, an allowance aimed at young hunters. That creates a strange situation, though, in which a semiautomatic rifle like the AR-15, which is more powerful than many handguns and has captured the imagination of so many mass killers, is available to a deranged 19-year-old while a pistol isn’t. Trump could try to sell a new age ban on those grounds, that he’s just closing the “loophole” by which an “assault weapon” is somehow legal for teenagers to buy while more pedestrian weapons aren’t. If hunters object, he could tweak the proposal so that only semiautomatic long guns are off limits to teens. If you want a hunting rifle when you’re 18, no problem.

Douthat was back on Twitter this morning defending his proposal from gun-rights advocates. It’s true that not all massacres are carried out with semiautomatic rifles (see, e.g., Virginia Tech) or by twentysomethings (see, e.g. Las Vegas). But:

Put another way, about 15 percent of the years of male adulthood (defined roughly as 18-70) produce more than 40 percent of mass killings. Maybe that's not a large enough disproportion to justify age-based regulation but it's still a disproportion (and an unsurprising one).

— Ross Douthat (@DouthatNYT) February 21, 2018

Four of the top five mass shootings in the US by body count did, for instance, and on this list of recent shootings you can see a greater deadliness in the semi-auto rifle cases:https://t.co/phyGdzCHGY

— Ross Douthat (@DouthatNYT) February 21, 2018

Of the last 25 mass shootings, nearly two-thirds (16) were carried out by men under 30. You’re not going to end mass murder by limiting young men’s right to buy semiautomatic rifles but you might prevent some. But if POTUS is understandably leery about taking on this project himself, he could just endorse the idea in principle and leave it to the states to raise age limits on the purchase of long guns, like a good federalist might. Right now only Illinois and Hawaii have made long guns off-limits to the under-21 group.

If you want to worry about Trump on guns, you may be better off worrying about how impressionable he is generally than about his interest in any particular proposal. The president is not a man known for his, ah, ideological rigor and it’s no secret that he pays a lot — a lot — of attention to what he sees on TV. Having students from Stoneman Douglas High on television all week was a canny move by the media and anti-gun advocates for that reason alone, even if it doesn’t move the needle of public opinion much. Trump is watching — and so are some of his aides in the West Wing who may themselves not be particularly ideologically rigorous. From Axios:

The “kids” part is important, and the images on TV have affected his conversation…

As he looks toward improving his standing in the suburbs ahead of his reelection race in 2020, nothing would give him more fawning from the “Morning Joe” crowd than taking on the NRA.

As Trump ponders this question, a few aides in the West Wing will wander down the corridor and tell him: “This is your moment. You’ll be a hero.”

Those aides’ names wouldn’t happen to rhyme with “Schnared” and “Schivanka,” would they? According to Politico, “One White House source told POLITICO that he and fellow administration officials were taken aback by the bump stock announcement” as they’d received no heads up that it was coming. That’s how quickly the president is capable of acting when he’s moved to action, whether by sympathy for the victims, by his own law-and-order inclinations, or what have you. Gun-rights supporters might want to pick up those phones!

Exit question: So excited have some gun-control fans been by the outspokenness of the Stoneman Douglas students that they’ve proposed … lowering the voting age to 16 because of it. If we’re going to lower the age to exercise one right while raising the age to exercise another, can we fiddle with the age for abortion too?