Associated Press

An AR-15 like this one was used in the Sandy Hook Elementary School massacre on Friday.

In 1994, a bipartisan group of lawmakers successfully approved a measure popularly known as the "assault weapons ban," which President Clinton signed into law. When it came to reauthorize the law in 2004, the Bush/Cheney administration was content to see the law die, and Congress took no action.

There's now renewed talk, at least among Democrats, of trying to revive the law, and President Obama is on record supporting the ban, though he's made no real effort to encourage Congress to act on the issue.

As the debate gets underway again, it's worth pausing to note that the assault weapons ban did not actually ban assault weapons, at least not all of them. Brad Plumer published a helpful primer this morning, refreshing our memories about what the law did -- and what it did not do. The trouble starts with the seemingly simple task of defining what an "assault weapon" is.

There’s no technical definition of an “assault weapon.” There are fully automatic weapons, which fire continuously when the trigger is held down. Those have been strictly regulated since 1934. Then there are semiautomatic weapons that reload automatically but fire only once each time the trigger is depressed. Semiautomatic pistols and rifles come in all shapes and sizes and are extremely common in the United States.

Congress didn’t want to ban all semiautomatic weapons — that would ban most guns, period. So, in crafting the 1994 ban, lawmakers mainly focused on 18 specific firearms, as well as certain military-type features on guns. Complicated flow charts laid it all out. Certain models of AR-15s and AK-47s were banned. Any semiautomatic rifle with a pistol grip and a bayonet mount was an “assault weapon.” But a semiautomatic rifle with just a pistol grip might be okay. It was complicated. And its complexity made it easy to evade.

This, naturally, led to loopholes that undermined the integrity and efficacy of the law.

For example, assault weapons that may have met the legal definition, but were made before 1994, were exempt from the ban, as were 24 million large-capacity magazines in existence before the law took effect. What's more, because the law was narrowly focused on 18 specific weapons, gun manufacturers found it easy to make modest modifications to existing models so that they would, technically, not fall under the prohibited list.

It will be extremely difficult for any proposed legislation to overcome Republican opposition, especially in the House, but even before those discussions can begin in earnest, proponents will have to take a long look at the original law's flaws before unveiling a viable alternative.