There's an opportunity for a small, positive step on gun control

Usually, when gun control supporters talk about "automatic weapons" after a mass shooting, there is a mass eye-roll from conservatives.

Automatic weapons - guns that fire multiple bullets when the shooter holds down the trigger - are very difficult to obtain. They've been heavily restricted by federal law for over 80 years, and their use in crimes is extremely rare.

Usually, weapons used in mass shootings are semi-automatic: A new bullet is automatically chambered after each shot, but the shooter must pull the trigger to produce each shot. This leads to a much slower rate of fire than a fully automatic gun can achieve.

But after the shooting in Las Vegas, the reaction was different. The extremely rapid fire the shooter was producing sure sounded like an automatic weapon, firearms experts said.

It's not yet 100% clear how the shooter achieved that rate of fire. But officials told The Associated Press they found two bump stock devices in the shooter's hotel suite. These are attachments that harness the power of a semi-automatic rifle's recoil to essentially bounce the trigger back and forth, causing the rifle to fire at a rate usually associated with a fully automatic gun.

Here's how it works: The shooter braces the rifle with his non-shooting hand and uses constant pressure to pull the gun forward. After each shot, the gun's recoil pulls the trigger away from his shooting finger, causing the trigger to reset. Then, the constant forward pressure from his non-shooting hand pulls the trigger back toward his trigger finger, squeezing off the next shot much more rapidly than the shooter could achieve with normal operation.

These devices are legal becuase they don't actually make a gun automatic. The shooter must still depress the trigger separately for each shot fired. But they produce the effect - extremely rapid fire - that is associated with automatic weapons.

These devices are mostly marketed as novelties, because they reduce shooting accuracy. But if you don't require accuracy, for example because you are shooting indiscriminately into a large crowd, they can make the weapon you're using more effective.

If automatic weapons are illegal, why should it be legal to make or sell or own or use a device that causes a semi-automatic weapon to behave like an automatic weapon?

Yes, if bump stocks are made illegal, gun owners might retrofit their devices illegally. But that will be harder, and result in a lower-quality modification, than if such accessories can be bought legally.

Laws against burglary haven't eradicated burglary, but we don't treat that as a reason to give up on stopping burglary, nor do we conclude that burglary laws have no effect on the burglary rate. Nobody says "if burglary is outlawed, only outlaws will burgle," because that would be stupid.

I am under no illusion that prohibiting bump stocks and similar devices would greatly reduce the rate of gun death in the United States. Most gun deaths are caused by handguns. This would not address handgun crime. But it could make it harder for an imitator who seeks to replicate the novel, long-distance mass-shooting technique that Stephen Paddock used - and therefore such a ban could save lives.

Mass shootings also terrorize the public, which is a reason to be concerned about showing the public that government is taking proactive steps to make them less likely, beyond the body counts themselves.

If Congress has time to worry about the huge problem of lottery winners using Medicaid, it has time to worry about devices that make it easier to kill more people in a mass shooting.

Many Democrats have adopted a tone of defeatism about gun control. Former Rep. Steve Israel of New York has an op-ed in the New York Times headlined "nothing will change after the Las Vegas shooting."

That kind of defeatism is not helpful.

The conversation after this mass shooting doesn't have to be the same as all the others. This shooter used a novel and terrifying technique that could be addressed with policies that weren't previously at the center of the gun control debate.

Republicans have said they're open to legislation regarding devices that make semi-automatics behave like automatics. Sen. John Thune of South Dakota, the third-ranking Republican in the Senate, told Bloomberg "to turn semiautomatic weapons into virtually automatic weapons, you know, that's something I think we'll take a look at."

Democrats should hold Republicans to that - take the information we learn over the coming days in the investigation and push for legislation to make it harder for someone else to do what this week's shooter did.

If Republicans block any action, then Democrats should complain. But first they have to try.