Gun manufacturer Smith & Wesson has donated $500,000 to the US’s largest gun industry trade body to help its fightback against the Massachusetts attorney general’s ban on the sale of assault rifles to civilians.

Smith & Wesson, whose shares this week hit a record high on the back of increasing demand for its assault rifles and other guns, said it had donated the money to the National Shooting Sports Foundation’s (NSSF) battle against “arbitrary government action that threatens to turn lawful gun owners and dealers into criminals”.

Maura Healey, the Massachuetts attorney general, last month moved to close a loophole that allowed the sale of assault rifles, which are illegal under the state’s assault weapons ban, after the firearms manufacturers had been given what Healey regards as “small tweaks”.

Healey said that following the horrific atrocity in Orlando, in which 49 people were killed and 53 injured at the hands of an attacker using an AR15-style rifle and a handgun, the state would no longer let gun manufacturers “exploit our laws”.

“The gun industry has found a way to exploit our laws, a loophole of potentially horrific proportions. And it’s time we act,” she explained in an impassioned editorial in the Boston Globe. “It’s no surprise the Orlando killer chose an AR-15 style assault rifle. It’s a weapon of war, originally created for combat, and designed to kill many people in a short amount of time with incredible accuracy. It’s in the same category as weapons chosen by killers in Newtown, Aurora, and San Bernardino. These are not weapons of self-defense. They are weapons used to commit mass murder. And they have no business being in civilian hands.”

Healey said the Massachusetts assault weapon ban, which mirrors the federal ban that expired in 2004, prohibits the sale of specific weapons like the Colt AR-15 and AK-47 as well as “copies or duplicates” of those weapons. “But gun manufacturers have taken it upon themselves to define what a ‘copy’ or ‘duplicate’ weapon is,” she said.

Healey said gun companies have been getting around the law by selling state-compliant “copycat versions of their assault weapons” with “small tweaks that do nothing to limit the lethalness of the weapon”.

“The gun industry doesn’t get to decide what’s compliant. We do,” she said at a press conference announcing the new ban, which went into force late last month.

The gun lobby fought back on Thursday, launching plans for legal action against Healey who it claims has “undermined the legislative and public process by unilaterally declaring products that were legal to be illegal”.

Lawrence Keane, NSSF’s senior vice-president and general counsel, said: “Her actions totally disregard 18 years of Massachusetts firearm law, supported by all state regulatory agencies and understood by all concerned, in which firearm retailers have operated.

“In doing so, she has endangered the livelihoods of family-owned businesses and made potential felons out of tens of thousands of law-abiding citizens.”

Keane said the NSSF has retained Michael Sullivan, a former acting director of the US Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) to prepare its legal filing.

The NSSF also called on gun owners and manufacturers to donate to its #GUNVOTE campaign, that helps gun owners to register to vote “and encourages them on election day, armed with the facts, to #GUNVOTE so they do not risk their rights”.

The NSSF praised Smith & Wesson for its “unprecedented half-million-dollar contribution”, which comes on top of donations from other assault rifle manufacturers.

“It is exactly that kind of commitment that will help ensure that our history and our rights will remain intact not just for us today, but for the generations to whom we will pass them on,” Keane said. “And it is an example of dedication to our industry we encourage others to follow.”

James Debney, the British chief executive of Smith & Wesson, said: “We are honored to support this effort on behalf of our employees and especially the law-abiding firearm owners of Massachusetts, who have so recently been denied their fundamental rights through arbitrary government action that threatens to turn lawful gun owners and dealers into criminals. To stop this from happening elsewhere, it is imperative that citizens across our nation are informed and knowledgeable about their rights, their candidates and the importance of their vote in this critical election year.”

Sig Sauer, the German gun company that made the SIG MCX used by the Orlando killer, donated $100,000. The company describes the Orlando killer’s gun as “ECLIPSING ALL THAT’S COME BEFORE IT” and the “result of years of toil and sweat, the SIG MCX is proof that the dawn of a new era doesn’t happen overnight”.

“We can’t imagine a more important time in our country’s history as it relates to many issues, not the least of which is the second amendment and our gun rights,” Ron Cohen, Sig Sauer’s president and CEO, said. “We believe that everyone must get out and vote this year, not only in the presidential election, but in many, many critical congressional races. Our future is still in our hands.”

Healey, who has suffered horrific sexist and homophobic abuse online since she introduced the new ban, said action must be taken to try to save lives. She said 10,000 assault rifles nearly identical to the one used in the attack in Orlando were sold in Massachusetts last year, and sales of the weapons jumped 450% in the wake of the Pulse nightclub massacre.

A person has tweeted that he would like to hire a homeless man to rape and disembowel the attorney general alive. Another is a said to have posted her home address on a gun blog, underneath a commenter has posted: “Road Trip???. Who wants to be a hero?”