The gun implicated in the Boulder shooting uses the same ammo as an AR-15. It’s legally a pistol.

In a promotional video for the Ruger AR-556, a company product manager gushed that the weapon, legally designated as a pistol but closer to an AR-15 rifle in design, offered much more than a typical handgun.

The compact body features the same rail system as wildly popular AR-15-style rifles, he noted, allowing owners to attach accessories like optics and flashlights, and its short barrel makes for easy transport.

“It’s still great for all the applications of a pistol - or a rifle,” the Ruger staffer said in the 2019 video.

Two key components that help define what a rifle is, the barrel and the stock, have been altered by firearm manufacturers to circumvent existing gun laws, critics have said, producing a weapon that functions much like a rifle but is legally classified as a pistol. That affects how they are bought, sold and regulated.

The result is the AR-15-style pistol. The weapon, championed by some gun enthusiasts but relatively obscure among the general public, has become more visible following the massacre Monday in Boulder, Colo., in which a gunman killed 10 people in a grocery store.

The suspect bought a Ruger AR-556 days before the killings, police wrote in an arrest affidavit. They noted that they recovered a protective vest, a rifle and a handgun but did not say whether either was the Ruger.

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What is an AR-15-style pistol?

AR-15-style pistols are a broad type of weapon that includes the Ruger. The firearms are modeled after semiautomatic rifles and have similar mechanisms.

The pistols even use the same ammunition and magazines as their AR-15-style rifle counterparts. That ammunition travels much faster and with more energy compared with common pistol rounds like the 9mm and are known to create more serious wounds, researchers and physicians have said.

AR-15-style pistols have much shorter barrels than typical rifles. For example, Ruger’s AR-556 pistol models have barrels between 9.5 and 10.5 inches, or about half-a-foot shorter than its rifle cousin. A shorter barrel will reduce some muzzle velocity.

The lack of a stock is another factor that lets AR-15-style pistols skirt the definition of a rifle. They are sold either with nothing attached or with a stabilizing brace that helps the operator secure the weapon to the forearm for one-handed firing, like a typical pistol.

However, the brace can also be used to shoulder the rifle like a regular stock, as shown in videos, making the pistol function essentially no different from a rifle.

The platform has been used in other mass shootings. A gunman in Dayton, Ohio, killed nine people in 2019 with an AR-15-style pistol.

Why do stocks and barrels matter?

Shorter barrels are favored in the military, law enforcement and elsewhere because they can easily fit into vehicles, lessen the chance of snagging on doorways and generally make a weapon more compact, said Doug Parisi, a former police captain. Some people use AR-15-style pistols as a “truck gun,” or a compact weapon to keep in a vehicle.

That compact size, especially of a rifle-type pistol, makes the weapon more easily concealed in a coat or a bag. And a barrel with six inches less physical space makes it harder for someone to grab or knock away when it is raised, compared with a typical rifle.

If a shooter had designs of entering and moving around a store, a compact weapon would be appealing, said Parisi, now a director of training at SafeDefend, which teaches safety courses, including active-shooter scenarios. However, witnesses told police the suspect shot people outside, making it less clear whether he intended to conceal it, Parisi said.

The National Firearms Act regulates rifles with barrels shorter than 16 inches more strictly than regular rifles because their potential concealment is a danger, according to a former special agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives. The 1934 law was aimed at gangland criminals who concealed short-barrel weapons, like the Thompson submachine gun and sawed-off shotguns.

Buying a short-barrel rifle requires a lengthy background check that could take months and includes fingerprints, a photo, purchase from a specialized dealer and a $200 tax, according to the former agent, who spoke on the condition of anonymity because of the sensitivity of the issue.

But AR-15-style pistols aren’t defined as short-barrel rifles.

“It’s treated just like a Glock,” the former agent said. “But it’s not just a Glock.”

Why do these exist?

There are some legitimate applications, Parisi said, including for security professionals who protect dignitaries or public figures and need something powerful but easily concealed and carried around. The weapons are popular in Latin America for that reason, he said.

The legal status of the pistol also helps eliminate hurdles when security guards in the United States travel across state lines that may regulate rifles differently, he said.

But for typical consumers, it’s a question of novelty of a different kind of gun and familiarity with the platform of the AR-15, which is the most popular gun in the country. Other gun enthusiasts may be drawn to AR-15-style pistols as an act of defiance of the federal regulations it is designed to challenge.

“It shows you can bend the rules regardless of how many rules are out there,” Parisi said.

What are regulators and officials doing about AR-15-style pistols?

The ATF in December sought to examine how braces, like the kind used with the Ruger, affect the classification of a firearm. The agency abandoned its proposal days later after a public outcry from gun advocates.

The city of Boulder passed an ordinance banning assault-style weapons in 2018, but it was overturned in court 10 days before the killings.

The language included restrictions for AR-15-style pistols, but is unclear whether the suspect purchased it in the city or elsewhere.