What can we do to prevent – or at least reduce the carnage from – mass shootings? As a public policy matter, the question is not difficult.

We can prohibit the possession of high-capacity gun magazines.

Madmen can easily kill many people because they are carrying guns loaded with large numbers of rounds. Gun magazines holding 30 rounds are common; some magazines hold 75 rounds or more. Gun magazines – or gun clips – are the devices that hold the bullets and feed them into the firing chamber. Some are built into the gun itself; most are detachable.

Someone with a modern, semi-automatic gun equipped with a magazine holding 30 rounds can fire 30 bullets before having to do anything to reload. If the magazine is detachable, he may then replace the empty magazine with a full one, and resume shooting.

A shooter armed with one gun and three detachable 30-round magazines can fire 90 rounds in less than a minute, even though he must twice replace the magazine. But if the shooter had magazines that held only five rounds each, he would have to replace the magazine 17 times to fire 90 rounds.

The federal assault weapons ban, which was enacted in 1994 and expired in 2004, restricted gun magazines to ten rounds. But while that was potentially the most important provision of the law, it was crippled by a grandfather provision exempting existing magazines. People who already had high-capacity magazines could continue to own and use them. Moreover, these existing magazines could even to be lawfully sold and purchased. People stocked up on high-capacity magazines before the legislation was enacted, making the existing stock of high-capacity magazines so great that the restriction was of little value.    

Senator Dianne Feinstein (D-CA) promises to introduce a warmed-over version of the 1994 assault weapons ban. Her proposal repeats the earlier mistakes of focusing too much on the cosmetics of assault weapons, limiting gun magazines to ten rounds rather than a lower number, and – worst of all – grandfathering existing weapons, including high-capacity magazines. To be effective, new legislation must apply to all guns and magazines. Legislation that allows people to continue to use – and even sell and purchase – previously-existing high-capacity magazines is of limited utility. Senator Feinstein wants to rely on background checks of purchasers of previously-existing high-capacity magazines, but most mass murderers have used lawfully-purchased weapons. Public safety demands that all high-capacity magazines be taken out of general circulation.

Moreover, while the 1994 federal assault weapon ban limited magazines to ten rounds, mass killings since that time – Columbine, Virginia Tech, Aurora, Newtown, and most recently the ambush of firefighters in Webster, NY, to name only the most notorious on a much longer list – demonstrate the need for a lower limit. Guns containing five rounds are adequate for self-defense, target practice, and hunting. The late Senator Barry Goldwater famously said: “If any S.O.B. can't hit a deer with one shot, then he ought to quit shooting.”

Gun magazines are simple devices and relatively inexpensive. For the good of the country, it is not to much to ask gun owners to replace their high-capacity magazines with new magazines holding fewer rounds.

Of course, high-capacity magazines should still be available to law enforcement personnel and the armed forces. But they should no longer be available to members of the general public.

Gun-rights advocates will argue that murders are committed by people, not guns, and that we should instead focus on mental illness. But the simple fact is that while it is possible to control guns, it is – in a free society – more difficult to control the mentally ill. We can’t always identify them; we can’t be sure which among them may become homicidal; and we can’t lock up everyone – in some cases indefinitely – who we suspect might be dangerous.

Restricting guns and gun magazines to five rounds will not solve all of America’s gun problems. On an average day in America, almost as many people are murdered with guns as were killed in Newtown, Connecticut (and if we include suicides using guns, more Americans die by guns on an average day than were killed in Newtown). We are less aware of this continuous daily carnage because it mostly involves individual shootings. Nor will a five-round restriction eliminate all multiple-victim murders. Madmen will still be able to arm themselves with multiple guns and magazines. But it will make carnage more difficult, and may even deter some madmen from their bloody plans entirely. To not do this because it will cure all of our gun problems would be like not producing medicine that would save half of the people with lung cancer because it would not save everyone.

Senator Feinstein may think it politically impossible to enact truly effective legislation, but there is unlikely to be a more propitious time than now, in the wake of Newtown, to do something meaningful. The major mistake of the gun control movement in the past has been to support fatally flawed legislation in the hope of strengthening it later, but that has proved to be a failed strategy. When we enact gun control legislation – with all the hoopla and self-congratulation that goes with the passage of new laws – we only mislead Americans into believing that we have done what we can. That removes gun control from public debate, and when the medicine proves to be ineffective, intensifies the belief that gun control doesn’t work.

UPDATE (January 6, 2012): This piece by Paul M. Barrett in Bloomberg/Businessweek is the best I've seen since Newtown.