Four of the nation’s top gun control advocates have joined the effort by President Joe Biden’s Bureau of Alcohol, Tobacco, Firearms and Explosives to regulate, tax, and eventually snuff out one of the most popular firearms in America, the AR pistol.

The groups said today that they filed a third-party brief in support of an appeal to a federal court’s earlier decision to ice a new rule from the ATF that would essentially ban the gun.

The Brady Center to Prevent Gun Violence, Everytown for Gun Safety Support Fund, Giffords Law Center to Prevent Gun Violence, and March For Our Lives Foundation said they joined to support ATF because they believe that AR-style pistols fitted with “stabilizing braces” are dangerous and have been linked to four mass shootings.

Yvin Shin, with March For Our Lives, said, “Stabilizing braces turn an already deadly weapon into an exponentially more efficient weapon of terror and mass destruction.”

Late last year, a federal district court judge in Texas put a nationwide hold on the ATF rule after it went into force. The judge said that ATF had overstepped its authority in targeting the braces, which have been in use for years to help handicapped shooters hold the large AR-style pistols.

The ATF rule faces several court challenges around the nation, and it is expected that the Supreme Court will have to settle the fight.

The ATF changed its view of the braces to say that they can be used as rifle stocks, turning AR pistols into short-barrelled rifles of the size that require taxes and regulation under the Al Capone-era National Firearms Act written to end gangland violence.

There are an estimated 20 million AR-style pistols in circulation. Under ATF’s new rule, those who don’t register those firearms and pay a $200 tax could face up to 10 years in prison.

The four groups argued that the ATF made the correct decision and said it would save lives.

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Biden has pushed for a complete ban on sales of all styles of ARs and AKs.

The gun industry argued that the ATF rule violated the Second Amendment and recent pro-firearms Supreme Court decisions. It also argued, and the district court agreed, that the ATF simply changed its mind without warning and after approving the braces for years.