The AR-15 assault rifle is a fearsome weapon, forged from aircraft-grade aluminum and as powerful as a Kandahar-bound M16. When Colorado shooting suspect James Holmes fired off as many as 50 shots a minute during his Friday-morning rampage, at least one bullet blasted all the way through the movie theater’s wall, hitting a victim on the other side. The AR-15 is also easy to shoot—“any idiot can do it,” a former border-patrol agent told CNN.

Among gun-rights advocates, the AR-15 appears to be popular. During her presidential campaign last December, Minnesota Rep. Michele Bachmann spoke highly of it. “My favorite gun is an AR-15 because you can be so accurate with it,” the congresswoman told WHO-AM radio interviewer Jan Mickelson. “And I scored the best in my class of any of the men too.” Asked by her interviewer, “And that’s a combat weapon?” the then-candidate responded, “I love it. It’s a great gun.”

Neither Bachmann nor her staff was available for comment this weekend.

There also exists a popular online “Colorado AR-15 Shooters Club” forum, with hundreds of thousands of posts. A thread expressing sympathy for the victims of the massacre had some 428 entries as of Saturday morning. One user wrote, “If news sources are checking us out, you know the government is as well. We need to be especially mindful of what we say and how we portray ourselves and firearms in general.”

But until eight years ago, Holmes would have had a lot of trouble buying the semiautomatic assault rifle in Colorado. Congress placed heavy national restrictions on the sale of assault weapons in 1994, making it “unlawful for a person to manufacture, transfer, or possess” an AR-15 and guns like it. The law also limited magazine size to 10 bullets.

Even while the ban was in effect, many of its provisions were skirted, as gun manufacturers simply modified their weapons or changed their names, and its effect on bringing down crime rates remains ambiguous. In 2004, after intense antirenewal efforts by the National Rifle Association and House Republicans, the Federal Assault Weapons Ban was allowed to expire.

Some states, such as New Jersey and California, have taken action at the state level to replace the federal law. In 2004, Mitt Romney, then governor of Massachusetts, signed such a bill banning assault weapons in that state. (Today he opposes all new gun-control laws.) During his presidential campaign in 2008, Barack Obama pledged to enact a new federal ban on assault weapons during his first term in office, but no such legislation has been introduced.

Even if the assault-weapons ban were still in effect, it would not have prevented Holmes from buying his other weapons (PDF): a rugged shotgun and a semiautomatic police pistol. In Colorado, it is relatively easy to obtain a permit to openly carry a firearm; while a concealed weapon permit is more difficult to get, they are also available. According to Time, Holmes did not have such a permit.

In order to purchase any of his four guns, Holmes would have had to undergo a background check. Colorado state law prohibits gun registration.

Though he relied mostly on the AR-15, Holmes also packed an 870 Remington 12-gauge shotgun, for which he purchased 300 rounds. Designed in 1951, it’s a hugely popular weapon: the bestselling pump-action shotgun in history, used by special forces from the United Kingdom to the U.S. Internal Revenue Service.

As the Remington company puts it, it’s a shotgun that “serve[s] the needs of law enforcement, the military, competitive shooters and people defending their homes and loved ones. When you’re in the most demanding environments and every second counts, Remington’s tactical shotguns give you the confidence and firepower to get the job done.”

In 2010, an anonymous artist painted a poster of a Rosie the Riveter–esque Sarah Palin holding a Remington 870 for Shotgun Magazine, an image widely distributed on gun forums. (Vice President Joe Biden prefers the high-end Beretta shotgun.)

Holmes’s other weapons were two .40-caliber Glock handguns, a standard-issue semiautomatic pistol that commands 65 percent of the police market. Holmes brought one into the theater, and left the other in his car.

A .40-caliber Glock would be the high-end of the power scale among the pistol’s variants—one version sells for $438. The Glock was designed in the late 1980s as a way to outmatch increasingly well-armed criminals. Today, in New York alone, 20,000 police officers carry one. Before it, enforcers as high up as the FBI relied mostly on revolvers. According to Paul M. Barret, author of Glock: The Rise of America’s Gun the lieutenant who spearheaded the Glock initative “kept a framed copy of the Second Amendment on his office wall.” The Glock has a light trigger, and is easy to aim and fire. “You pull it out, you pull the trigger, and you put it away,” said the officer. “That was the beauty of it.”

Last year, Jared Loughner used a Glock to shoot Rep. Gabrielle Giffords and 18 others in Tucson, Ariz. Giffords herself told The New York Times in 2010, “I have a Glock nine-millimeter, and I’m a pretty good shot.”

After finishing his massacre, Holmes walked out to the movie-theater parking lot, and reportedly began idly firing at his car. Though he had already fired hundreds of shots, he still had plenty of ammunition left.