Taking handguns away from law-abiding civilians can endanger them — that’s a familiar argument. What most people might not know is that many police organizations also oppose handgun bans because of the harm they can cause to police training and preparedness.

That’s what we argued this month when we filed a friend-of-the-court brief against the District of Columbia’s handgun ban in the U.S. Supreme Court. One of us, Ed Nowicki, is head of the International Law Enforcement Educators and Trainers Association, a professional association of police trainers. We were joined in the brief by the other major police firearms training organization, the International Association of Law Enforcement Firearms Instructors.

The lawful availability of handguns for citizens provides the police with a much larger pool of recruits who have experience with handgun safety, and who have learned basic familiarity or developed proficiency in the use of handguns. That’s why our brief was joined by a broad coalition of law enforcement organizations, including the Maryland Fraternal Order of Police.

A citizen who has experience with handguns in hunting or target shooting will have acquired the habit of keeping his finger off the trigger until the last instant before the shot. For a police officer, this is a life-or-death skill; staying off the trigger while drawing the weapon in an emergency prevents accidental shootings.

Likewise, a police recruit who has enjoyed target shooting as a civilian will have learned how to hold a handgun with a strong but not over-tight grip, and how to keep the gun steady while firing, avoiding the muzzle flip that causes missed shots.

There are only so many hours in a police academy for firearms training. If a trainee is picking up a handgun for the first time, he will have to spend time acquiring elementary familiarity with its operation — flipping the safety on and off, and reloading quickly. It takes a while for these actions to become second nature, and that time would be better spent refining already developed skills, such as practicing engagement with multiple targets.

Another amicus brief, filed by a group of retired generals and admirals, makes a similar point about military training and describes research showing that military recruits who have firearms training in civilian life perform much better in combat.

Many innovations in police firearms training have been created by civilian trainers, who often have more time to dedicate to the subject than do police instructors; many police instructors do not teach fulltime, and those who do must teach a variety of subjects. For the same reason, many police firearms instructors are civilians.

Jeff Cooper, a civilian, invented The Modern Technique, which is now the standard model for defensive pistolcraft. Mr. Cooper created the technique after World War II, based on his observations that Army handgun shooting methods were far from optimally effective. Many thousands of police officers — and police trainers — learned their skills at Mr. Cooper’s Gunsite Ranch in Arizona. Other civilian experts, such as Massad Ayoob of New Hampshire and John Farnam of Colorado, have also made important contributions to handgun doctrine, and have themselves trained many police officers and police instructors.

The widespread civilian possession of handguns also helps the police do their job. In countries such as the United Kingdom (where handguns are banned) or the Netherlands (where handguns are rare), the home-invasion burglary rate is 48 percent to 59 percent, and many home-invasion burglaries lead to assaults or rapes.

In the United States, only 13 percent of house burglaries take place when someone is home, and studies show this is because about half of U.S. homeowners have a gun. And handguns are by far the best guns for home defense, because they’re easy to maneuver in confined spaces and hard for criminals to grab.

Because there are so many fewer home invasions in America, there are many fewer emergencies to which the police must respond. Thus, the police have more resources available to thwart, investigate or deter other crimes.

Where law-abiding people are allowed to have guns, criminals know they’re rolling the dice.