The 10-page complaint argued that certain assault-style weapons are allowed under federal law after Congress in 2004 let a ban expire. Plaintiffs said restricting access to those guns in Massachusetts — which has had its own state-level ban since 1998 — would violate equal protection, interstate commerce and firearms rights sections of the U.S. Constitution.

Business at Mass Armament “is, and has been built up, centered around firearms and large-capacity magazines banned by the Act,” the complaint said, estimating that about 70 percent of the shop’s business will be lost under the new limitations.

It’s the second lawsuit filed in recent months opposing the major new law Gov. Maura Healey signed in July, which seeks to crack down on untraceable ghost guns, prohibits firearms in some public places, expands the state’s “red flag” law that allows authorities to remove firearms from someone deemed a threat to themselves or others, and more.

Another group of residents and the Gun Owners Action League sued on Aug. 1, focusing their complaint largely on the overhaul of gun licensing requirements. The law requires Bay Staters to obtain a “firearms safety certificate,” which involves completion of a live-fire training, to secure a license to carry.

Plaintiffs in that case alleged the new training requirements “are impossible to meet at the present time,” arguing that no such live-fire training courses existed and no regulations had been promulgated.

But about six weeks after they filed their suit, the outlook changed.

On Sept. 12, House and Senate Democrats suddenly announced they reached agreement on a mid-year spending bill and shipped it to Healey hours later. The final 34-page measure was packed with additional policy reforms, including one section postponing the new firearms safety certificate language until 18 months after the gun law took effect.

The requirement to pass a live-fire training course to obtain a license to carry now will not begin until April 2026, according to Attorney General Andrea Cambell’s office.

Attorneys also pointed to a Sept. 30 memorandum that the state Executive Office of Public Safety and Security sent to law enforcement agencies, licensing authorities, firearms dealers and other parties, which emphasized that local authorities “should continue to accept and process applications for [firearms identification cards] and [licenses to carry] in the ordinary course” until the changes start.

Campbell’s team on Friday asked a federal judge for about another month to produce a formal response to the GOAL-backed lawsuit, writing that attorneys need to work with plaintiffs to figure out how much of the complaint remains “operative” after the legislative action.

“Until the effective date (in April 2026), applicants will be permitted to obtain a [license to carry] — provided they meet all other requirements for licensure, see G.L. c. 140, § 131 — under pre-existing requirements, which do not include a live-fire training requirement,” Campbell’s office wrote. “In light of these recent developments, it will be necessary for the parties to confer to determine how much of the complaint remains operative, and which claims remain to be addressed by Defendants through an answer or motion to dismiss. The parties therefore need additional time to discuss the manner in which the case will proceed.”

Jim Wallace, executive director of the Gun Owners Action League, said plaintiffs intend to keep their lawsuit active in the meantime.

“We need to make sure that regardless of suspending the new criteria, are the [police] chiefs recognizing that the old criteria are still in place?” he said. “It seems that [lawmakers] can change their mind at will with all of this stuff. It’s very important to keep the lawsuit in place because they may change their minds next week.”

Lawmakers did not call attention to their change to the gun law at the time they unveiled and approved the supplemental budget.

House Speaker Ron Mariano’s office said the provision sought to amend a drafting mistake in the gun law, which negotiators filed around 6:30 p.m. on July 17 and the House and Senate muscled through the following afternoon.

The final line of that 115-page bill declares that Section 38, which deals with training for licensed gun sellers, and Section 75, which concerns data-collection about firearms used in crimes, take effect 18 months after the remainder of the law.

A line in the September spending bill changed the gun law by applying the 18-month delay to Section 74, not Section 75

Wallace argued that the apparent error reflects flaws with the underlying process.

“This thing wasn’t vetted. The public wasn’t allowed to give input on the final bill. The Legislature didn’t even get a chance to read the final bill before they passed this. Certainly, the state agencies were not ready for any of this. There’s no systems in place to handle anything in the new law and there’s no money appropriated,” he said. “This was all done in a rush on purpose because they knew the bill could not stand the light of day. That’s why they rushed to pass it.”

Gun safety groups have widely praised the law as a major step forward to keep rates of gun violence low in Massachusetts, especially as law enforcement agencies grapple with the increasing prevalence of ghost guns.

Wallace said he expects additional lawsuits will be filed to challenge other portions of the law.

A coalition of Second Amendment supporters is also pursuing a 2026 ballot question that would seek to repeal the law. The group must file at least 37,287 signatures with local elections officials by Wednesday, and an organizer said last week volunteers already collected more than 65,000.

The coalition plans a press conference Tuesday outside the State House to make a “major announcement.”

Healey last week issued an emergency preamble to the law, which allowed most of it to take effect and prevented opponents from suspending it until voters decide the final fate in more than two years.