Shortly after Adam Lanza shot and killed 20 grade-schoolers and six adult staff members at Sandy Hook Elementary at Newtown, Conn., in 2012, the National Rifle Association’s executive director, Wayne LaPierre offered this mantra: “The only way to stop a bad guy with a gun is with a good guy with a gun.”

In other words, the armed citizen — equipped with a concealed weapon or an openly carried handgun — will stop an assailant long before the cops arrive.

You would not expect a fair test of that premise in California, where the state gives local law enforcement discretion to issue concealed carry permits and where open carry laws are heavily restrictive. Ohio allows concealed carry licenses and permits open carry.

But in Texas, the NRA has persuaded lawmakers to liberalize concealed carry and open carry laws with almost as much success as they’ve enjoyed in Idaho.

So how did it work out Saturday when a suspect identified as 21-year-old Patrick Crusius used a variant of an AK-47 to kill 22 people at a Walmart in El Paso?

Here’s what former Housing and Urban Development Secretary, San Antonio Mayor and presidential candidate Julian Castro told NBC’s Chuck Todd Sunday:

“This happened in Texas, a state that has one of the highest rates of gun ownership. It has concealed carry. It has open carry. The shooter knew he would be walking into a store where a lot of people would be carrying a gun. That did not deter him.”

Why not?

Training, for starters. In a crisis, you might freeze with disbelief or shock. Or you’re just as likely to flee for safety. To overcome that impulse, police and military personnel undergo exhaustive training involving moving, acting aggressively, concealment and targeting the suspect.

Even after all that, it’s only a simulation. The school security officer at Marjory Stoneman Douglas High School at Parkland, Fla., is facing prosecution for failure to act when a former student murdered 17 classmates last year.

Even if you have to undergo training for a concealed carry permit — and Idaho no longer requires a permit for anyone 18 or older — the most strenuous examination could involve firing a weapon at a target from a fixed position.

Firepower, for another. A handgun with a 4-inch barrel isn’t much of a match — at least in terms of accuracy — against a military assault rifle.

There’s always timing. Gun advocates such as LaPierre maintain you can’t wait on the cops. Response takes too long.

That wasn’t true at the July 28 Gilroy, Calif., Garlic Festival shooting. It took police about a minute to respond.

Cops were on the scene at Dayton, Ohio, even more quickly on Sunday.

At El Paso, however, six minutes elapsed between the first 911 call and law enforcement arriving on the scene.

So the odds are that by the time an armed citizen has time to react, the police will arrive.

And speaking of the armed citizen, this question remains unanswered: In a crisis, how are the cops supposed to tell the “good guys with a gun” from “a bad guy with a gun”?

So what are the options?

There’s Occam’s razor. Eliminate the obvious threat. Ban assault rifles. Impose high fines — even up to the point of civil forfeiture — for possessing one illegally. Ban high-capacity magazines. Ban the ammunition that was designed to kill military combatants from the civilian market.

Not interested?

Then you’re left with a couple of extremes:

Arm everyone entering a Walmart, a community festival or a night club with an AR-15.

Or start doing all your shopping online. Just have them leave your groceries at the back door. — M.T.

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