After a mass shooting, Americans hone in on the specifics to try to prevent the next one: Was he mentally ill? Did he use a bump stock? Is 18 too young to buy an AR-15?

Democrats and Republicans, and President Donald Trump, are considering the last question. A bill in Congress would raise the age from 18 to 21 to purchase a semiautomatic rifle; the legislation, if it had been in place, might have prevented a former student from killing 17 people at the school in Parkland, Florida, he had attended.

A measure seeking a reduction in gun deaths is a worthy cause, but the age increase proposal won’t lead to sweeping change. Since 2009, men under 21 committed two mass shootings with a semiautomatic rifle, according to the gun control group, Everytown for Gun Safety. Only one of the two guns used was purchased legally, meaning only one shooting (where four or more people were shot and killed) would have been prevented.

“The frustration that I feel as someone working in this area is that following this tragic mass shooting, the only thing people want to think about is this latest mass shooting,” said Daniel Webster, a professor who works with the Johns Hopkins Center for Gun Policy and Research.

“What our lawmakers need to be thinking about is,” he said, “of course we need to address school shootings, but how are they similar to other types of shootings? And what policies can address all of them?”

Few teenagers commit mass shootings with semiautomatic rifles

According to information collected by Everytown for Gun Safety, since 2009 there have been 156 mass shooting incidents — defined as four or more people shot and killed, not including the shooter.

Eleven mass shootings have been committed by men 21 and younger, and two of those (or 1.3 percent of the total) were committed using an assault rifle. Meanwhile, only one of those rifles were purchased legally.

A semiautomatic rifle was used in 11 of the 156, or 7 percent of events.

While two mass shootings might have been prevented by the change, this reform does nothing about the other 154.

We’re not wrong to focus on age. We’re just addressing the wrong weapon.

Legislators in support of the bill — including Sen. Dianne Feinstein, who wrote it, and Sen. Jeff Flake, who is gathering bipartisan support — point to laws prohibiting those under 21 from buying handguns to explain the “common sense nature” of the bill. But that’s not entirely accurate.

A kid too young buy a handgun should be too young to buy an #AR15. Working with @SenFeinstein on a bipartisan bill that will raise the minimum purchase age for non-military buyers from 18 to 21 - the same age you currently have to be to purchase a handgun. — Jeff Flake (@JeffFlake) February 21, 2018

While you have to be 21 to purchase a handgun from a licensed dealer, the age limit drops to 18 to purchase one from a private, unlicensed seller. This means 18- to 20-year-olds can still buy firearms from online retailers, at gun shows, or even from a friendly neighbor, a loophole that’s had disastrous consequences.

“Far, far more young people ages 18 to 20 commit violent crimes with handguns than they do assault rifles,” Webster said.

Everytown’s research puts the rate at which 18- to 20-year-olds commit homicides at nearly four times higher than adults 21 and older.

“We’ve got to start talking about the handgun component here, it’s just crazy,” Webster said.

This isn’t the same as raising the drinking or the smoking age

When the drinking age went up to 21 in an effort to prevent drunk driving, the rate of people dying from drunk drivers really did decrease dramatically.

A 2014 review of the research published in the Journal of Studies on Alcohol and Drugs showed that although many young people disobey the drinking age, it has decreased drinking and has saved lives.

As Vox’s German Lopez explains, “The review found the drinking age saves at least hundreds of young lives annually just as a result of reduced alcohol-age-related traffic fatalities among underage drivers.”

Can’t buy beer? Can’t buy an #AR15 — Geraldo Rivera (@GeraldoRivera) February 22, 2018

What’s another option?

Looking across the spectrum of mass shooting data, the most relevant factor is ammo/magazine capacity, or how many times an active shooter is able to fire a gun before he needs to reload it.

While the name “semiautomatic” might seem to indicate a high magazine capacity, there’s actually no correlation between the two: Automatic refers to the loading mechanism, not the firing capability.

Webster advocates laws that limit large capacity magazines. He also recommends looking to California as a model, which has started treating ammunition sales similarly to gun sales. The state now requires ammo be sold from a licensed dealer and, starting in July 2019, will require background checks.