Following a deadly mass shooting in Boulder, Colo., House Democrats are calling on President Joe Biden to “take Executive Action to regulate concealable assault-style rifles,” the type of weapon used by the shooter. Four Democratic representatives — Gun Violence Prevention Task Force Chairman Mike Thompson (D-Calif.) and Vice Chairs Joe Neguse (D-Colo.), Val B. Demings (D-Fla.), and Ed Perlmutter (D-Colo.) — authored a letter to Biden that was then signed by 101 House colleagues.

“For too long, gun manufacturers in order to circumvent the National Firearms Act have designed and marketed concealable AR-15 style firearms which fire rifle rounds,” the lawmakers wrote. “Concealable assault-style firearms that fire rifle rounds pose an unreasonable threat to our communities and should be fully regulated under the National Firearms Act consistent with the intent and history of the law.”

Lawmakers want the Ruger AR-556 pistol, the weapon used by the shooter, to be subject to the same restrictions as short-barrel rifles. The National Firearms Act regulates short-barrel rifles differently from other rifles since they can be more easily concealed, a former special agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives explained to the Washington Post.

But currently, the Ruger AR-556 is not regulated like a rifle under the National Firearms Act (even though it can fire rifle rounds) because it is classified as a pistol under the law. To purchase a rifle similar in size to the Ruger AR-556 would require a background check, fingerprinting and numerous forms of ID as well as a waiting period. But none of that is currently required for the Ruger AR-556, which the shooter purchased 6 days before the incident.

As Kris Brown, president of Brady: United Against Gun Violence, told NPR, a prior national assault weapons ban put into place by former president Bill Clinton helped reduce mass shootings, but it lapsed in 2004. “When we had a ban on [AR-15 and other assault-style] weapons that Joe Biden helped put into effect [while he was in the Senate], we saw a 25 percent reduction in gun violence associated with those kinds of slaughters,” she said.

Since the ban ended, the United States has seen escalating numbers of mass shootings, according to Stanford University researcher John Donohue, who also spoke with NPR. “You saw more mass shootings in the five years than you had seen in the previous 10 years. So things are not only getting worse since the Federal Assault Weapons Ban ended, but they’re getting worse faster,” he said.

The city of Boulder attempted to implement its own assault weapons ban in 2018 that likely would have blocked sales of the Ruger AR-556 pistol used by the shooter, but the local ban was overturned in court just 10 days prior to the recent mass shooting, thanks in part to a National Rifle Association lawsuit.