Former congresswoman Gabrielle Giffords (D-Ariz.) listens as her husband, Mark Kelly, speaks Monday on Capitol Hill. (Susan Walsh/AP)

The leader of a prominent gun-control advocacy group said Friday that the willingness of the National Rifle Association and congressional Republicans to consider new restrictions on an accessory used in Sunday's massacre in Las Vegas represents a potential watershed moment.

"It is an acknowledgment by the Republicans that a law does matter — that laws matter, period," said Mark Kelly, co-founder of Americans for Responsible Solutions and husband of Democratic former congresswoman Gabrielle Giffords, whom a gunman badly wounded in a 2010 attack.

Kelly spoke to reporters a day after the NRA said that "devices designed to allow semi-automatic rifles to function like fully-automatic rifles should be subject to additional regulations."

Law enforcement officials say that Las Vegas shooter Stephen Paddock appears to have used "bump stocks" — devices that allow legal semiautomatic rifles to mimic the rapid fire of a fully-automatic machine gun — during his assault at a country music concert.

In the aftermath of the attack, the White House and multiple Republican lawmakers have expressed openness to taking action to restrict bump stocks, and bills addressing the issue are expected to be introduced in both the House and Senate. The focus on the particular accessory has taken attention away from what has become the standard demand for Democrats and gun-control advocates in the wake of mass shootings: expanded background checks, as well as restrictions on military-style rifles and high-capacity ammunition magazines.

Kelly, who appeared with Giffords outside the Capitol on Monday to call for legislative action, said those bills remain priorities. But he said a bump-stock ban would be a clear advance for his movement, undermining the NRA talking point that new gun laws do not keep Americans safe and that, in the well-publicized words of NRA Executive Vice President Wayne LaPierre, "the only way to stop a bad guy with a gun is with a good guy with a gun."

"We'll try to use every opportunity we have to pass the legislation that really matters," Kelly said. "Most Americans do not die from semiautomatic long guns with bump stocks on them. . . . Inner-city violence and domestic violence takes the lives of a lot of people, so that's where the focus needs to continue to be. But the fact that the Republicans will at least acknowledge that laws matter is a big deal to us."

It is an open question, however, whether bump stocks will be addressed through legislation. The NRA and some lawmakers are pressing the Bureau of Alcohol, Tobacco, Firearms and Explosives to take regulatory action against the devices — reversing rulings confirming their legality made during the Obama administration.

That action, in the eyes of Republican leaders, could leave the need for congressional action moot and avert a potentially messy legislative battle. But Kelly warned that Congress should act regardless, warning that an administrative ruling could have broader implications that can be addressed only through legislation.

"It would be better if Congress passed legislation and the president signed it, certainly," he said, pointing to the need to clarify what should happen to the bump stocks already on the street. In 1986, Kelly noted, Congress effectively outlawed the private possession of newly manufactured automatic weapons, and existing ones were tightly tracked and regulated.

"We don't want people to have automatic weapons that we don't know who they are, they're not registered. These people aren't fingerprinted. That is not a good scenario," he said. "We need to put these people in the position that the thing that they have, that they are now holding something that is illegal for them to be in possession of."

Peter Ambler, executive director for Americans for Responsible Solutions, said that the NRA was "very, very loose in their statement" about bump-stock regulation, and he expressed doubt that ATF under President Trump would craft an airtight ruling.

"These are not individuals, institutions inside the current administration, that we trust to keep us safe," he said. "Congress should absolutely respond to this."