Handguns were used in more than 80 percent of gun murders each year, but gun control advocates had failed to interest enough of the public in a handgun ban. Handguns were the weapons most likely to kill you, but they were associated by the public with self-defense. (In 2008, the Supreme Court said there was a constitutional right to keep a loaded handgun at home for self-defense.)

Banning sales of military-style weapons resonated with both legislators and the public: Civilians did not need to own guns designed for use in war zones.

On Sept. 13, 1994, President Bill Clinton signed an assault weapons ban into law. It barred the manufacture and sale of new guns with military features and magazines holding more than 10 rounds. But the law allowed those who already owned these guns — an estimated 1.5 million of them — to keep their weapons.

The policy proved costly. Mr. Clinton blamed the ban for Democratic losses in 1994. Crime fell, but when the ban expired, a detailed study found no proof that it had contributed to the decline.

The ban did reduce the number of assault weapons recovered by local police, to 1 percent from roughly 2 percent.

“Should it be renewed, the ban’s effects on gun violence are likely to be small at best and perhaps too small for reliable measurement,” a Department of Justice-funded evaluation concluded.

Still, the majority of Americans continued to support a ban on assault weapons.

One reason: The use of these weapons may be rare over all, but they’re used frequently in the gun violence that gets the most media coverage, mass shootings.