It sounds like a big, unexpected deal: A lopsided bipartisan House majority will vote to ban a category of firearms this afternoon. In fact, the modest measure is the handiwork of Second Amendment champions, and its passage probably guarantees no expansion of gun control gets enacted in the year since the Sandy Hook schoolhouse massacre.

The legislation would extend for a decade a longstanding prohibition on the production of entirely plastic weapons. Most Republicans will support keeping the law on the books, describing their vote as evidence of the reasonableness in the gun violence debate. Most Democrats will vote “yes” as well, because the bill stands to be the only gun control measure getting close to the House floor for the foreseeable future.

But most Democrats would like the bill to do more, and most Republicans want to make sure nothing more regarding firearms gets on the legislative docket. The suspense is over which side gets its way once the bill arrives in the Senate. The mystery won’t last long, because the 1988 law is set to expire next week.

Charles E. Schumer, D-N.Y., the leading proponent of gun control in the Senate, has been pushing an alternative bill that would extend the statute for just one year, which he says would allow sufficient time to update the law for the coming age of sophisticated 3-D printing and the ever-expanding roster of high-tech materials.

Schumer tried to put such a bill before the Senate before Thanksgiving, but was thwarted by GOP objections. If he insists on pursuing that approach when senators return Dec. 9, a lapse in restrictions on plastic guns is guaranteed, because that Monday is also the expiration date on the law. And even if Schumer could get his more expansive version through the Senate — which looks like a long shot — it would then face some delay and a problematic future back in the House.

The current law requires that all guns be manufactured with some metal components, to ensure they can be detected by X-ray machines at airports, courthouses and other high-security places. Some plastic guns have been designed so they can be fabricated with readily detachable metal pieces, making them easy to disassemble and sneak past security checkpoints. Advocates of gun control want the law rewritten to say that the metal parts on plastic guns must be non-removable, which would make such weapons more difficult to make with the help of 3-D printers.

Firearms makers and their Republican allies say plastic weapons are a tiny niche that hasn’t grown to become a legitimate public safety threat, and say that changing the statute would burden the gun industry.

Given the impending deadline, the complexity of the back story and the eagerness of so many lawmakers to claim a few modest achievements before the end of the year, the Second Amendment crowd looks likely to prevail in the argument by ceding to the small bill. And if that happens, it will mark an undefeated streak for gun rights advocates in a year that was supposed to be the most challenging for them in decades.

Dec. 14 marks the anniversary of when 20-year-old Adam Lanza — armed with weapons obtained legally — shot and killed his mother, then 20 children and six adult staff members at Sandy Hook Elementary School in Newtown, Conn. He then killed himself. It was the second deadliest mass shooting by a single person in American history. But an intense effort by President Barack Obama to win expanded background checks and curbs on big ammunition clips foundered in the Senate in April. Additional efforts — including a campaign orchestrated by former Rep. Gabrielle Giffords, D-Ariz., the victim of a 2012 assassination attempt — have led nowhere.