A man holding a Colt AR-15 semi-automatic rifle. KAREN BLEIER/AFP/Getty Images

If some aspects of the attack at the Pulse gay nightclub in Orlando Sunday morning appear either unique or unusual — the death toll was the highest for a single mass shooting in U.S. history and the Florida venue was known as a safe space for the lesbian, gay, bisexual and transgender community — there are other ways in which it had much in common with other high-profile mass killings in recent years.

Omar Mateen, the shooter slain by police, employed a military-style rifle that is similar to the weapon of choice of many mass shooters across the country.

And, like many of those killers, Mateen didn’t appear to have any problem getting his hands on one.

Nine days ago, Mateen, who had been investigated by the FBI on two occasions, walked into a gun store near his home in Port St. Lucie and bought an assault-style rifle, initially reported as an AR-15 but later identified as a Sig Sauer MCX. The next day, he came back and picked up a Glock 17 pistol. Last Thursday, he returned for large magazines of ammunition so he could fire multiple rounds. Wielding the weapons just a few days later, Mateen entered the Pulse nightclub and opened fire, killing 49 people over a few hours and wounding even more as law enforcement personnel scrambled outside to respond to and stop the carnage.

While assault rifles aren’t used as frequently in everyday gun crimes as other types of weapons, they are responsible for some of the highest death tolls in mass shootings.

Assault-style rifles can cost about $1,200 or more, commonly hold about 30 rounds in a magazine and be accurate as far as roughly 500 yards. A single pull of the trigger results in just one bullet exiting the firing chamber, unlike military assault rifles like the fully automatic M-16. Although AR-15s and Sig Sauer MCXs are commonly called assault rifles, the technical definition of that term implies a fully automatic weapon that can spray numerous bullets per trigger pull.

The large magazines used in assault-style rifles and the speed with which they can be exchanged make the rifles a go-to weapon for civilian shooters looking to spray a room with bullets. In fact, the style of weapons were used to inflict massive damage by all the assailants in the mass shootings at the Pulse nightclub in Orlando Sunday; the Inland Regional Center in San Bernardino, California, last December; the Umpqua Community College in Roseburg, Oregon; and the Sandy Hook Elementary School in Newtown, Connecticut.

While data on the exact number of assault-style rifles existing in the U.S. are difficult to come by — one estimate indicated 1.5 million of them were privately owned in 1994. Another estimate suggested there 1.7 million were manufactured from 1986 to 2007 and not exported. Finally, the National Rifle Association (NRA) estimated in 2013 that Americans owned about 5 million AR-15s. Clearly, this style of rifle appears to be a big component of the firearms industry.

Recent estimates have indicated that the sales of assault-style rifles are growing much faster than those of other types of guns. Among the top 15 gun manufacturers in the U.S., 11 of them feature an assault rifle model. And they are marketed with “heroic” and “patriotic” imagery, according to a study conducted by the Violence Policy Center, a gun control advocacy group.

The National Rifle Association reported the firearm’s popularity has soared in popularity among gun owners recently. The rifle is in demand because it is adaptable, customizable and reliable, as well as accurate in hunting and self-defense situations, the NRA blog said. The assault style of weapon is available at retailers in much of the country, including the shop in Port St. Lucie, where Mateen bought his gun. Some retailers list the guns online.

There hasn’t been a federal ban on assault-style rifles since 2004, and comparatively few states have laws restricting them. Just eight states have bans on the high-powered weapons that have been used in recent mass shootings, and one of them, Hawaii, only prohibits assault pistols.

If assault-style rifles are so effective in carrying out mass shootings, why are they legal?

At least to some degree, the answer to that question is the NRA, which spent a record $3.6 million last year lobbying the federal government in opposition to gun control measures, according to data reported by the Center for Responsive Politics. The association also contributed almost $1 million to political candidates, mostly Republicans, in the latest election cycle, CRP data show.

The NRA’s lobbying muscle and the money it pumps into candidate campaign coffers enabled it to push for the 1994 assault weapons ban to be dropped by a Republican-controlled Congress in 2004. It also succeeded in the 1990s in killing federal funding for research into the dangers of gun violence, leading to a present-day lack of research by the federal government into whether lax gun laws increase or decrease safety.

It appears unlikely the situation will change anytime soon. The NRA — which President Barack Obama has repeatedly attacked for keeping him from implementing gun control measures that could curb the number of shootings during his tenure — enjoys the backing of a Republican Party that controls both houses of Congress. The next man or woman who wants to legally buy the same type of assault-style rifles used Sunday will likely have as easy a time getting it as Mateen did, unless reforms are made, something the president acknowledged in his statements after the shooting rampage.

“Today marks the most deadly shooting in American history. The shooter was apparently armed with a handgun and a powerful assault rifle. This massacre is therefore a further reminder of how easy it is for someone to get their hands on a weapon that lets them shoot people in a school, or in a house of worship, or [in] a movie theater, or in a nightclub,” Obama said. “And we have to decide if that’s the kind of country we want to be. And to actively do nothing is a decision as well.”

This story has been updated to clarify the specific type of weapon used by the Orlando shooter.