President Obama on Wednesday gave Vice President Joe Biden Jr. a month to complete a job that he could have finished that afternoon. It is time to come up with, as Mr. Obama put it, “a set of concrete proposals” to make the nation safer from guns. The ways to do this are well-known because the nation has grappled with gun massacres many times before. It is Congress that hasn’t.

For years sensible gun-control bills have been offered and rejected. The occasional bill has actually become law — but in hollow, loophole-riddled form — and then been allowed to lapse. Farther-reaching proposals focusing on things like banning certain kinds of bullets, or taxing them out of existence, have been laughed at.

Many of the good ideas, some expressed on this page this week, involve sensible limits on who can buy guns and how they can be sold. Mr. Obama should also focus on the weaponry itself, starting with restoring — after toughening — the ban on assault weapons that expired in 2004. Assault weapons are versions of military rifles that are meant to kill people, not paper targets, clay pigeons or deer. They account for only a fraction of the guns sold and used in the United States, but they play a hugely outsize role in the national slaughter; rampage killers love them.

The expired ban was shredded with loopholes, which gun dealers easily exploited. The rifle Adam Lanza carried in Newtown, Conn., a semiautomatic Bushmaster, was a version of the AR-15, a widely popular form of the military’s M-16 and M-4. But it was not illegal, even in Connecticut, which outlaws assault weapons, because it differed from banned weapons in cosmetic details, not in lethality. A revived assault-weapons bill should have stricter definitions to capture more of these lethal weapons than before. Senator Dianne Feinstein, the California Democrat who sponsored the original ban, has promised to reintroduce it early in the new year. In the House, Representative Carolyn McCarthy, a Democrat of New York, has a bill containing what should be an element of any law Mr. Obama proposes: a ban on magazines that hold more than 10 rounds of ammunition. High-capacity magazines allow someone to commit mass murder in seconds, or minutes, without the inconvenience of having to reload.