Nicolas Cruz bought his assault rifle and ammunition legally in Florida, despite these facts: He had been expelled for disciplinary reasons; he was considered too dangerous to wear a backpack in school; he had been in and out of mental health treatment; he posted on Instagram photos of his arsenal and of himself in a terror mask.

No law will stop all gun violence, and no law can abridge citizens’ fundamental Second Amendment rights. But our Constitution never was intended to give monsters like Cruz some “right” to kill; rather it provides for a “well-regulated” militia. So here are five regulations we need:

• Federal licensing: A federal license to purchase and carry firearms would supersede all state or local permits and would be required for every person buying or carrying any firearm. Available to law-abiding citizens, such a license would be granted within 30 days after application, renewable every five years, and valid throughout our country.

Restrictions should be reasonable, as we have in Massachusetts: no criminal background; no history of serious mental illness; no open restraining orders; modest proficiency (range test showing basic safety, reasonable accuracy); and sign-off by the local chief of police.

With such a federal license, law-abiding citizens could legally transport guns for hunting, target shooting or protection and legally buy and sell guns anywhere in the U.S. — all well-regulated, without fear of interstate conflicts or confiscation.

• Eliminate all loopholes, including gun shows: Anyone buying or trading guns would need a federal license, whether at a gun shop, a gun show, a sporting goods store or in a personal transaction. Just as one must present a driver’s license when renting or buying a car, all sellers would be required to produce a verifiable federal license.

• Control large-capacity firearms: “Large-capacity” refers to any firearm holding more than nine rounds, or any detachable magazine with a capacity of more than eight rounds. As a lifelong hunter, I can attest that nobody needs more than nine shots to kill game. Nor does anyone who knows how to shoot need more than nine rounds for protection. For targets, skeet, trap and sporting clays, one doesn’t need even that many. To be clear, the owner of a gun that can accept magazines with a capacity of eight or more rounds who does not own any such magazines would be within the law.

An enthusiast who just has to have large-capacity firearms could undergo much deeper background checks and get a federal special assault weapon license. It should be easy for a law-abiding citizen to get a standard federal license, but not an assault weapon license. We must be extra careful to whom we license guns designed for mass killing.

• Every person holding any large-capacity firearm or magazine would have two years to do one of the following: trade it in to the Bureau of Alcohol, Tobacco, Firearms and Explosives for full retail value (taxpayer funded so gun owners are made whole); register it as a “collect-only” firearm, requiring them to be kept under lock and key but not used without an assault weapon license; or obtain an assault weapon license.

• No citizen could possess a working fully automatic weapon, and owning one would be a felony. Anyone converting a semi-automatic to a fully automatic weapon would go to jail.

These five regulations would not stop every mass shooting. But lives would be saved when the most lethal firearms are kept from the most dangerous people. Yes, some outlaws would still have guns. But far fewer, and it would become easier to track and convict the criminals who use them.

Meanwhile, the Second Amendment rights of millions of law-abiding citizens like me will be protected. Law-abiding gun owners like me don’t need loopholes. We have nothing to hide. We don’t covet military-grade weapons for slaughtering people. Just like every American, we abhor these horrible massacres of innocent souls, of children.

— Harry Van Sciver lives in Marstons Mills.