Dean L. Winslow, a retired Air Force colonel and flight surgeon, is a professor of medicine at Stanford University.

Last week, I withdrew my name from consideration to be assistant secretary of defense for health affairs. My appointment had been put on indefinite hold by the Senate Armed Services Committee, and I felt the Defense Department needed to fill the position without undue delay. Our soldiers, sailors, airmen and Marines deserve the full complement of Pentagon appointees to support them.

I am sorry not to be able to assist Defense Secretary Jim Mattis, whom I deeply respect, in building the best and most efficient military health-care system possible. I have the credentials to help, including 35 years of experience in the Air Force (including four deployments to Iraq and two to Afghanistan after 9/11), in military and academic medicine, and in private practice, public hospitals, the Department of Veterans Affairs, the pharmaceutical and diagnostics industries, and public health. But unfortunately, I do not possess one credential the committee wanted to see: I do not support the unrestricted ownership of semiautomatic assault weapons by civilians.

I arrived in Washington for my hearing the day after the mass shooting in Sutherland Springs, Tex. I heard about it when I got off the plane from England, a country with effective gun control and minimal gun violence. As a devout Christian, a parent, a doctor and an American, I was deeply troubled by yet another loss of innocent lives, this time in the sanctity of a house of worship.

At my hearing, jet-lagged and in mourning over those killed, I was asked by Sen. Jeanne Shaheen (D-N.H.) about the military discharge status of the shooter. He had been given a bad-conduct rather than dishonorable discharge and — despite episodes of domestic violence and a stint in a psychiatric hospital — was able to buy several weapons, including an AR-15-style gun. Although I stated that discharge status was not my area of responsibility, Shaheen pressed her point. I acknowledged that the Air Force dropped the ball and recommended an inspector general's investigation focusing on systems failures that contributed to the tragedy.

Then, I blurted out what was in my heart: "I'd also like to . . . just say how insane it is that in the United States of America a civilian can go out and buy a semiautomatic weapon like an AR-15." Sen. John McCain (R-Ariz.) interrupted, warning this was not in my "area of responsibility or expertise." Soon after, my confirmation was put on hold.

I am a marksman, rated expert in both the M-9 pistol and the M-16 rifle (the fully automatic military version of the AR-15). During one of my tours in Iraq, I spent hours with my Special Operations forces colleagues who were training Iraqi teams on our base, firing an array of military weapons. Using a powerful gun at a firing range is a real blast, and I support civilians experiencing that thrill at licensed ranges.

However, as commander of an Air Force hospital in Baghdad during the surge, I have seen what these weapons do to human beings. The injuries are devastating. In addition, because of their high muzzle velocities, assault weapons are challenging for untrained civilians to control and are not optimal for home defense. A pump-action 12-gauge shotgun, with its excellent stopping power, would be far better. Even with imperfect aim, a shotgun will hit its target, while the pellets won't go through a wall to endanger someone in the next room. Assault rifles are also poor hunting weapons due to low accuracy beyond 100 yards.

But with a standard 30-round magazine, assault rifles are perfect for mass murder. From 1995 to 2004, assault weapons were severely restricted in the United States. During that time, mass shootings were far less frequent — 1.6 compared with 4.2 per year after the ban lapsed in 2005. The experience in Australia is even more dramatic: No mass shootings have occurred there since assault weapons were outlawed in 1996. Assault weapons in the United States are not being used to kill "bad guys" in self-defense or to provide for a "well-regulated militia" but for entertainment, mass murder and domestic terrorism. Is this really the intent of the Second Amendment?

To be sure, my testimony was impolitic. At Senate hearings, the best responses reveal little to nothing. I know, too, that at least one other issue (my support for access to therapeutic abortion, in accordance with federal law, for women in military service was also a sticking point) helped torpedo my appointment.

Nevertheless, I have no regrets. Having semiautomatic weapons makes no sense. It is a public-health issue that, as a doctor, I felt compelled to bring to the Senate's attention. As a citizen, I am saddened that our government has become so dominated by pro-gun lobbyists that an appointment such as mine — which has no responsibility for gun control — can be sidelined by a single sentence of informed, personal opinion. And that really is insane.