The son of one Boulder mass shooting victim filed a lawsuit against Sturm, Ruger & Co., the maker of the pistol used in the March 2021 assault in the city’s south-side King Soopers that killed 10 people.

Nathaniel Getz, the son of 59-year-old Suzanne Fountain, alleges in the new case that Ruger used deceptive marketing practices by giving its AR-556 pistol capabilities to essentially function as a rifle while being regulated as a pistol.

The pistol, designed in 2019 as a variant of Ruger’s 2014 AR-556 rifle, uses the same ammunition and magazines as AR-15 guns but uses an altered barrel and stock to avoid federal classification as a rifle, according to the lawsuit.

The case was filed March 10 in Connecticut, where the manufacturer is based. It accuses Ruger of marketing the AR-556 varieties as “entry-level” AR-15-style guns and promoting them as weapons specifically for killing people.

The Denver Gazette reached out to a Ruger spokesperson for comment.

The pistol version of the AR-556 has some differences from its rifle counterpart that are key to how it’s regulated. It comes equipped with a collapsible butt stock, which Ruger calls a “pistol stabilizing brace.” The company not calling it a stock makes a regulatory difference.

The brace — which allows the gun to rest on the shooter’s forearm — “allowed its weapon to function as a stock-stabilized AR-15 rifle, while evading regulations targeted at limiting AR-15-style rifles,” the lawsuit claims.

The accused shooter bought the pistol legally less than a week before the massacre, court papers have revealed. He pleaded guilty to third-degree assault as a high school senior for beating up a fellow student he claimed called him racist names. But the conviction didn’t bar him from purchasing a gun.

Alleged Boulder shooter's misdemeanor not enough to bar weapon purchase

Buyers have to pass a background check, as with other pistols. But owners do not need a license to carry it in Colorado, unless they want to carry it as a concealed weapon.

By contrast, owners of short-barreled rifles need a federal license as civilian “collectors,” and the process comes with paperwork, a $200 tax, a visit from a local agent from the Bureau of Alcohol, Tobacco, Firearms and Explosives and an extensive background check.

Weapon in Boulder massacre slides under federal regulations

Lawsuits against gun-makers for the harm their products cause have generally been banned under a controversial 2005 federal law that shielded them from liability, the Protection of Lawful Commerce in Arms Act.

The law, however, has exceptions. The Connecticut Supreme Court ruled in 2019 that gun-maker Remington could be sued under an exemption by families of Sandy Hook victims over how it marketed its Bushmaster XM15-E2S rifle. The U.S. Supreme Court declined to hear Remington’s appeal of that ruling in 2019, and the company settled with the families for $73 million.

Prosecutors can't use own psychological expert for accused King Soopers shooter, Boulder judge rules

The victims killed on March 22, 2021 include: Denny Stong, 20; Neven Stanisic, 23; Rikki Olds, 25; Teri Leiker, 51; Suzanne Fountain, 59; Kevin Mahoney, 61; Lynn Murray, 62; Jody Waters, 65; Tralona Bartkowiak, 49, and Boulder Police Officer Eric Talley, 51.

The accused shooter faces 115 counts in connection with the massacre, including 10 charges of first-degree murder, counts related to possessing banned high-capacity magazines and 47 crime-of-violence sentence enhancers.

The Associated Press contributed to this report.