Here's what you need to know about the guns some are calling "the gold standard for mass murder," after Orlando shooter Omar Mateen used the assault-style rifle Sig Sauer MCX to kill at least 49 people. (Editor's note: This video has been updated with more specific information.) (Gillian Brockell/The Washington Post) 
 
 In the hours after Sunday’s mass shooting at an Orlando night club, Chief John Mina of the Orlando Police Department said the gunman’s weapons included a pistol and an “AR-15-type assault rifle.” 
 
 On Monday night, officials clarified that the rifle Omar Mateen used in the shooting was not an AR-15, but a Sig Sauer MCX rifle. 
 
 While aesthetically similar to and just as lethal as an AR-15, the MCX is internally a different beast, thus all but removing it from the AR-15 family of rifles. Yet while the weapon is different, the MCX and the AR-15 share the same design purpose: providing a highly portable, customizable, easy to operate and accurate rifle for the individual who possesses it. 
 
 [A lawmaker plans to give away assault rifles as fundraiser prizes days after Orlando] 
 
 As Bob Owens from the blog “Bearing Arms” points out, the MCX is a modular rifle designed to be able to change between a variety of calibers and “otherwise has no major parts that interface with AR-15s in any way, shape or form.” 
 
 Originally designed for U.S. Special Operations forces, the MCX was built from the “ground up” to be lightweight and short, and it accepts “a broad array of accessories, enabling you to build a complete weapon system for any scenario or environment,” Sig Sauer’s website says. 
 
 While able to shoot the same caliber ammunition — .223 — as an AR-15, the MCX was initially meant to fire a round called a .300 Blackout. The relatively new caliber, according to Owens, was designed to provide Special Operations Command with a bullet that was as quiet as a pistol round but packed the range and lethality of a rifle cartridge. A side effect of the bullet’s design is that the it mimics the size of the round fired by AK-47-type rifles. 
 
 The key difference, however, between the standard AR-15 series of rifles and the MCX is the operating system used to mechanically propel the bullet from the gun and cycle the next round to be fired. The initial AR-15 (AR standing for Armalite Rifle), designed by Eugene Stoner in the late 1950s, used a system called “direct impingement.” Most modern AR-15-type rifles use this system. 
 
 Direct impingement means that when a round is fired, the gas generated from the fired bullet is shot through a hole in the barrel and is then channeled back through a tube that leads directly to the weapon’s bolt assembly. The gas, proceeding back to the rear of the rifle, moves the bolt assembly to the rear, subsequently ejecting the spent shell casing. A spring in the stock of the weapon then propels the bolt assembly forward again, inserting another bullet from the magazine into the rifle’s chamber, thus restarting the whole process. 
 
 The MCX, however, is known as a “piston gun,” meaning it uses gas piston technology to operate the internals of the rifle. When the rifle is fired, instead of the excess gas going into a small tube that is directed to the bolt assembly, the gas is pushed into a cylinder that is connected to a piston. The gas in the cylinder then propels the piston, which then cycles the bolt much like the impingement system. Mikhail Kalashnikov’s AK-47 series of rifles also uses a gas piston system. 
 
 1 of 25 Full Screen Autoplay Close Skip Ad × What the investigation into the Orlando mass shooting looks like View Photos Officials are scouring the site and others in the area for information about the June 12 massacre. Caption Officials are scouring the site and others in the area for information about the June 12 massacre. June 15, 2016 Law enforcement officials gather outside Pulse nightclub in Orlando during the shooting investigation. David Goldman/AP Buy Photo Wait 1 second to continue. 
 
 Read more: 
 
 Shooter Omar Mateen’s father says he’s saddened by massacre, calls gunman ‘a good son’ 
 
 The history of the AR-15 
 
 This is the military-style equipment used to respond to the attack in Orlando ||||| by: Debbie Lord, Cox Media Group National Content Desk Updated: Jun 14, 2016 - 12:24 PM 
 
 After the attack in Orlando Sunday, and others that have taken place in the recent past, it seems we hear the terms “assault weapon” and “assault rifle” quite a bit. 
 
 Some would argue that with nearly 50 people killed in Orlando, the names of the weapons don’t matter and are simply a difference without a distinction. But when it comes to the two (separate) types of weapons, there is a great deal of difference. 
 
 So what is an assault weapon, how does it work and how is it different than an assault rifle? Here’s a quick look. 
 
 What is an assault weapon? 
 
 An assault weapon is a rapid-fire, magazine-fed rifle designed for military use. It is a shoulder-fired weapon that allows the shooter to select between semiautomatic (requiring you pull the trigger for each shot), fully automatic (hold the trigger and the gun continuously fires) or three-shot-burst mode. 
 
 What is an "assault rifle?" 
 
 Technically, there is no such thing. What’s called an assault rifle in reports on gun violence is actually a semi-automatic rifle that looks similar to the assault weapons used by the military. An AR-15 rifle, like one that has been used in some mass shootings, is an example of this type of weapon. According to law enforcement authorities, the shooter in Sunday's attack used a Sig Sauer MCX. 
 
 What’s the difference between a semi-automatic and an automatic weapon? 
 
 An automatic weapon (assault weapon) can shoot more than one round when you pull the trigger. A semi-automatic weapon (“assault rifle”) does not. 
 
 Automatic weapons have not been used in recent mass shootings. In the shootings in Orlando, New Town, Ct., and San Bernardino, semi-automatic weapons, one requiring you to pull the trigger each time to fire a cartridge, were the weapons used. 
 
 The weapon used in some of those shootings was an AR-15. Doesn’t “AR” stand for “automatic rifle”? 
 
 No, “AR” in the gun’s name stands for ArmaLite Rifle. That is the company that first developed the weapon nearly 60 years ago. The company sold the rights to the firearm to the Colt company which, in turn, modified the rifle and sold it to the military as the M-16. The M-16 is an automatic weapon – hold the trigger and multiple rounds can be fired. The AR-15, as does the Sig Sauer, requires that you pull the trigger to fire each cartridge. 
 
 If it is not an automatic weapon, how does a shooter fire so many shots? 
 
 Semi-automatic rifles can accommodate high-capacity magazines – compartments that hold cartridges. That allows the shooter to fire off tens of rounds in a short period of time. A Sig Sauer and an AR-15 magazine generally holds 20- 30 rounds. There are magazines that can hold more. 
 
 How tough is it to get a semi-automatic weapon? 
 
 In most cases, it’s no tougher to get a semi-automatic rifle than it is getting a handgun. Seven states -- California, Connecticut, Hawaii, Maryland, Massachusetts, New Jersey and New York – along with the District of Columbia have enacted laws banning semi-automatic weapons. Minnesota and Virginia regulate semi-automatic weapons. There is no ban on purchasing the weapons in any other state. 
 
 In Florida, you do not need a license to own or purchase a handgun, shotgun or rifle, nor do you have to register a gun. To purchase a gun from a gun store, you must pay $8 and complete the paperwork for a background check. If you pass the background check, you get the gun. If the gun is a rifle or a shotgun you do not have to wait three days to get it. For a handgun, there is a mandatory 3-day “cooling off” period in Florida – one of only 10 states that require any waiting period for the purchase of a gun. There is no federal waiting period required when purchasing a gun. Waiting periods are imposed by states. 
 
 Can I buy an automatic weapon? 
 
 Here’s where you need to understand the difference between automatic and semi-automatic. Semi-automatic weapons are relatively easy to purchase. However, the only automatic weapons legal to purchase for civilians in the United States are the ones that have been registered between 1934 and 1986. According to the Bureau of Alcohol, Tobacco, Firearms and Explosives, an “assault rifle” (or automatic weapon) is a machine gun. No new machine guns can be made or sold to civilians. Automatic weapons have been heavily regulated since the National Firearms Act of 1934. Taking possession of such weapons requires paying a $200 federal transfer tax, filling out an application to register the weapon, submitting passport photos, getting your chief law enforcement official to sign your application, and submitting to an FBI background and fingerprint check. Those weapons are hard to come by and generally pretty expensive. You do not want to violate the Firearms Act. If you do, expect to sit in prison for 10 years as you figure out how to pay the $100,000 fine. 
 
 Sources: The NRA; smartgunlaws.org; The FBI; The Wall Street Journal 
 
 © 2016 Cox Media Group. ||||| Omar Mateen, the Orlando shooter, used an AR-15-style semi-automatic rifle and a 9 mm semi-automatic pistol to kill 49 people at the Pulse nightclub Sunday. The same style of rifle was also used by the shooters at San Bernardino, California; Umpqua Community College in Oregon; Sandy Hook and several other mass shootings in recent years. On Monday, Democratic presidential candidate Hillary Clinton described the rifle as a “weapon of war” that should be banned, and President Obama concurred. But an effective ban could be difficult to devise. 
 
 The AR-15 used to be illegal. President Bill Clinton’s assault weapons ban, which was in effect from 1994 to 2004, banned the AR-15 and other guns that were too similar to military-style weapons. However, this law did not prohibit Americans from owning semi-automatic weapons; it capped how many military features an individual gun could have. During the ban, a semi-automatic rifle like the AR-15 could legally have any one of the following features, as long as it didn’t have two or more of them: a folding stock (making the gun slightly easier to conceal), a pistol grip (making the weapon easier to hold and use), a bayonet mount, a flash suppressor (making it harder to see where shots are coming from), or a grenade launcher. 
 
 A 2004 report commissioned by the Department of Justice on the effects of the assault weapons ban concluded that the law was largely ineffective at limiting access to weapons with the power of the AR-15. According to the report, the ban focused on “features that have little to do with the weapons’ operation, and removing those features is sufficient to make the weapons legal.” The report noted that several semi-automatic rifles were functionally equivalent to the AR-15 and untouched by the ban. It’s hard to know whether Mateen’s AR-15-style weapon would have been covered by the old ban, though some versions of the Sig Sauer MCX rifle he used are sold with more than one of the components that were limited by the law. Depending on how many military-style features the rifle had when he bought it, it might have been legal under the assault weapons ban. And he would have been able to modify the gun himself, even under the old law. 
 
 The review for the DOJ concluded that bans on specific models or features of assault weapons had little to no discernible impact on gun deaths. If the law had any effect, the report said, it was most likely the result of bans on large-capacity magazines, which contain 10 or more rounds. (Large magazines allow shooters to keep firing without pausing to reload, a point at which their targets could run or fight back.) Calculations based on homicide reports in Jersey City, New Jersey, suggested that restricting large-capacity magazines might lower the number of gunshot victims by up to 5 percent. However, there are a huge number of high-capacity magazines already in circulation. The report authors concluded that a ban on them probably wouldn’t make it hard to keep a determined shooter from legally buying a pre-ban magazine and pairing it with an AR-15 equivalent. ||||| These crawls are part of an effort to archive pages as they are created and archive the pages that they refer to. That way, as the pages that are referenced are changed or taken from the web, a link to the version that was live when the page was written will be preserved.Then the Internet Archive hopes that references to these archived pages will be put in place of a link that would be otherwise be broken, or a companion link to allow people to see what was originally intended by a page's authors.The goal is to fix all broken links on the web . Crawls of supported "No More 404" sites. ||||| ORLANDO, Fla. -- It took us 38 minutes to walk out of a gun shop with a death machine. 
 
 Just two days after Omar Mateen used a semi-automatic rifle to murder 49 innocent people and injure dozens more, we were able to purchase an AR-15 -- a rifle similar to Mateen’s that has been used in several other mass shootings on American soil -- in less time than it takes to buy a cart full of groceries. 
 
 We would have been locked and loaded within five minutes, but the gun shop employee we spoke to said the queue on background checks was longer than usual because people were scrambling to buy AR-15s in the wake of the shooting. That’s both terrifying and unsurprising -- gun sales often skyrocket after mass shootings. 
 
 But the ease with which we purchased our AR-15, a Vietnam-era gun hailed by the military as being the perfect killing machine, was surreal. This, despite the fact that Florida Gov. Rick Scott had declared a state of emergency in Orange County, which includes Orlando. 
 
 On any given day, regardless of whether your town has endured a horrific tragedy, it’s not hard to purchase a weapon of mass murder like we did in most of America. A reporter for the Philadelphia Daily News was able to get one in just seven minutes in Pennsylvania on Monday. 
 
 Within seconds, we had it in our hands: A rifle that weighs less than a small watermelon and can, like the MCX Sig Sauer that Mateen used, easily fire several dozen rounds in less than 10 seconds. Our Diamondback DB15's killing power is striking, and at $644, it’s a relatively affordable murder weapon. 
 
 Huffington Post HuffPost's Andy Campbell gets the feel of the Diamondback DB15. 
 
 Gun shop employees were happy to discuss lethality. When another customer looking to purchase a small, .22-caliber handgun noted that it wouldn’t fare well against a “6-foot-3, 350 pound man,” the employee agreed. 
 
 “You’d have to hit him in the neck or the heart,” said the employee, whom we aren’t naming for this story. 
 
 He went on to joke with the customer that he should either pick a bigger gun or use the .22 to “shoot him in the stomach 15 times, that’d really piss him off.” 
 
 Behind him, the TV blared Fox News coverage of the massacre at Pulse nightclub, located nearby. On the wall, a framed photo of President Barack Obama with the caption, “Firearm Salesman Of The Year,” hung next to an array of other long rifles, some of them higher caliber than the AR-15, and some of them more affordable, too. 
 
 Semi-automatic rifles like the AR-15 have been used in several of the most deadly shooting rampages in the United States in recent years, even before the attack in Orlando. The killers behind the mass shootings in San Bernardino, California, the elementary school at Newtown, Connecticut, and the movie theater in Aurora, Colorado, all used AR-15s. 
 
 Huffington Post 
 
 Andy Campbell/Huffington Post 
 
 They’re not the cheapest rifles around -- another, higher-caliber carbine hung on the wall at our shop for half the price. But the National Rifle Association calls the AR-15 the most popular weapon in the U.S. "It is customizable, adaptable, reliable and accurate," the organization says on its blog. 
 
 Shop workers told us we’d have to wait if we wanted to buy a smaller gun. There’s a three-day waiting period for handgun purchases in Florida, but that rule does not apply to rifles. 
 
 The employee made some small talk, declaring that all guns fall under one of three classifications: Leisure, defense and hunting. We aren’t sure which category Mateen’s gun falls under. 
 
 Under any of the gun shop employee’s definitions, there seems no reason for a weapon that can fire 10 or more bullets in quick succession. The AR-15 and its close cousins are not made to hunt deer. They’re made to kill people. 
 
 All it took to attain one was a driver’s license (the one we used was from out of state) and a few minutes to wait for the background check to clear. We signed state-issued proof of sale documents that asked whether we were felons or drug addicts, and where we lived. 
 
 Waiting for the background check was the hardest part -- and that only involved a few minutes of idle time. We requested data from the state Department of Law Enforcement on the current volume of background checks for similar guns, but did not get an immediate response. The department has been overwhelmed by public records requests since the shooting, a spokeswoman said. 
 
 Nothing that we or the store employees did was illegal or unusual. You can buy semi-automatic rifles at several stores in the Orlando area. 
 
 
 
 As of this writing, we have the same firepower that Mateen had when he opened fire at one of Orlando’s most famous gay clubs. We don’t have any ammunition, though -- the shop ran out of .223-caliber rounds. 
 
 We plan to return the gun. Eventually, it will join countless other rifles of its kind on the legal, Great American gun market. 
 
 UPDATE: 6/15 -- We attempted to return the AR-15 to the shop where we purchased it. Despite having a return policy, they refused. So we opted to turn the gun over to Orlando police, who said they would have it destroyed. ||||| ON JUNE 12th Omar Mateen killed 49 people and injured 53 with a 9mm handgun and a semi-automatic gun called the Sig Sauer MCX, an AR-15-style rifle trumpeted by its manufacturer as “the first true mission-adaptable weapon system” which “eclipses everything that came before it”. Despite having been under FBI investigation in 2013 and 2014, Mr Mateen faced no obstacles when buying these weapons in the days before he walked into Pulse, a gay nightclub, in Orlando, Florida, and sprayed revellers with bullets. Many commentators are wondering anew why such a dangerous weapon is freely available for civilian purchase. The AR-15 has been the gun of choice in several other recent mass shootings, including the killing of 14 people attending a holiday party last December in San Bernardino, California; the murder of 12 in a movie theatre in Aurora, Colorado in 2012; and, later that year, the slaughter of 26 children and staff members at an elementary school in Newtown, Connecticut. 
 
 The National Rifle Association (NRA) muses that the “AR” could stand for “America’s rifle”, as the AR-15 is the nation’s most popular long gun, with as many as 10m units in circulation. People gravitate to the AR-15 for hunting, sport shooting and self-defence, the NRA says, because “it is customisable, adaptable, reliable and accurate” as well as lightweight and easy to modify. Whereas its fully-automatic military cousin, the M16, shares its looks, the AR-15 shoots only one round per trigger-pull. It is thus a misnomer, gun-rights advocates say, to call the AR-15 a military-style “assault rifle”, a term that has been in use since Nazi Germany introduced the Sturmgewehr in 1944 (which in turn inspired the Soviet AK-47). Unlike fully automatic rifles, which can fire 750 to 900 rounds per minute and are available only to the armed forces, the AR-15 and its kin (sold by the millions to the general public) can muster only about 45 to 60. But as the Orlando tragedy showed, this circumscribed capacity can still be highly lethal. 
 
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 Gun-control advocates began using the term “assault weapons” to refer to semi-automatic firearms like the AR-15 in the 1980s. In the wake of a school shooting in Stockton, California, the first President Bush approved a measure to prohibit importing such guns in 1989. In 1994, Bill Clinton signed the Federal Assault Weapons Ban, a law restricting the manufacture of large-capacity magazines and a large range of semi-automatic firearms. But due to a ten-year sunset provision, the ban was lifted in 2004 and efforts to reinstate it—along with attempts to pass other gun-control measures—have failed. Gun-rights advocates complain that the “assault” moniker is meaningless. It is used by gun-shy activists, a National Review article says, as “a sharp and effective tool with which the enemies of the right to keep and bear arms have sought to sow confusion, fear and ignorance”. The guns falling under the “assault” umbrella are “a set of quotidian weapons that have never posed much of a problem to anybody”. 
 
 After another mass shooting in which an AR-15 most certainly posed a problem to many people trapped inside a nightclub, such a blasé approach to powerful civilian weapons may be harder to maintain. Ludwig Wittgenstein, a great 20th-century philosopher and a veteran of the first world war, wrote in his “Philosophical Investigations” that when “we employ the word ‘meaning’,’’ we are most often referring to the everyday purpose of some phrase. “The meaning of a word”, he wrote, “is its use in the language”. By that sensible if trivial standard: if an AR-15 can be used to assault large crowds of people, killing and maiming scores of them, on multiple occasions, it qualifies as an assault weapon. ||||| Editor’s note: This story focused on the AR-15 rifle, which was the weapon used in several recent mass shootings. Orlando law enforcement officials initially said this was the same gun type used in the Pulse nightclub shooting, but later clarified that the Orlando shooter was using a Sig Sauer MCX. The Sig Sauer MCX is a rifle with several similarities to the AR 15-style and was originally designed for use by U.S. special operation forces. The NewsHour regrets the confusion.​ 
 
 GWEN IFILL: The weapon used in the Orlando shootings is at once one of the most popular and most reviled weapons in America, the AR-15, the civilian version of the U.S. military’s standard issue rifle. 
 
 John Yang has a closer look. 
 
 JOHN YANG: The NRA says it’s America’s most popular rifle, used legally and safely by millions of people. A lawyer for victims’ families says it’s the gold standard for mass murder of innocent civilians. 
 
 The AR-15 has been used in some of the nation’s worst mass shootings, at the Sandy Hook Elementary School in Newtown, Connecticut, a movie theater in Aurora, Colorado, an Oregon community college, and now an Orlando gay club. 
 
 To learn more about the rifle, its popularity among gun enthusiasts and its place in American culture, we went to a Northern Virginia gun range to speak with former Deputy Assistant Defense Secretary Phillip Carter, himself a gun owner, who knows the AR-15 well from his service as an Army officer. 
 
 Why is this weapon so popular? 
 
 PHILLIP CARTER, Former Defense Department Official: So, the AR-15 is America’s rifle because it’s what America’s military carries. 
 
 It’s modeled on the M-16 that’s been carried by America’s Army and America’s Marine Corps and the rest of our services since Vietnam. Today, roughly 20,000 troops carry a similar rifle in Iraq and Afghanistan, and even those who don’t serve feel that they’re part of that effort when they carry the AR-15. 
 
 JOHN YANG: I have read that the recoil is a bit gentler than some other rifles. 
 
 PHILIP CARTER: So, the recoil on the M-16 or the AR-15 is designed to be light, so that you can keep it on target and continue to shoot bullet after bullet after bullet. It’s by design. 
 
 There’s a massive spring here in the stock that absorbs quite a bit of the recoil. There’s also, by design, a gas system that takes a lot of the gas from the firing of each bullet, and it evaporates it, so that when each bullet is fired, that gas and that recoil isn’t coming back on the shoulder each time, but it’s actually in the rifle in a more constructive manner. 
 
 JOHN YANG: And that feature would also allow a shooter to keep steady. 
 
 PHILIP CARTER: It’s a military rifle. It’s designed to deliver masses of bullets to a very specific target over time. This is a weapon designed to kill in mass quantities. It’s a military weapon. 
 
 JOHN YANG: So this is essentially a civilian version, you said, of the M-4 or the M-16. 
 
 PHILIP CARTER: Yes. This is very much like a civilian version of a Humvee or a civilian version of some other type of military gear. There are some modifications to this that have been made to make it a civilian variant. 
 
 What’s different on the civilian versions is, it only allows semiautomatic fire. So you flip this to fire, and it allows you to shoot one shot at a time. The military versions would allow you to switch to semiautomatic and then, once more, either for three-round bursts or for fully automatic, if you’re shooting an older variant of the M-16. 
 
 JOHN YANG: Fully automatic, meaning? 
 
 PHILIP CARTER: You squeeze the trigger, and the weapon keeps firing until the magazine runs dry. Your ability to put bullets down-range with this weapon is limited mostly by your ability to reload it. 
 
 And the magazine for this rifle looks like this. This is a 30-round magazine that carries 30 bullets of the 5.56-millimeter variety. And it goes into the magazine well right here. It’s very easy to reload. It’s very easy to get more ammunition in there and continue to shoot at your target. 
 
 The ammunition for the M-16 or the AR-15 is a 5.56-millimeter bullet. It’s a very small and very fast bullet that does a lot of tissue damage when it hits a person. So just the tip here is the bullet. It’s roughly the size of your pinkie tip. But it’s propelled forward by a massive cartridge of powder that shoots this at a very high velocity, creating a bullet that can punch through targets, whether they’re human or otherwise, with a great deal of lethality and power. 
 
 The other thing about this bullet is, because it’s so small and moves so fast, it tends to tumble or become volatile when it hits a person, and so it tends to create a very wound and very difficult-to-treat wounds. Again, it’s a military weapon. It’s not designed for hunting, where you might want to preserve the meat so that you can eat it. It’s designed to wound or kill soldiers in combat. 
 
 JOHN YANG: There are hundreds of companies in the United States that make verges of the AR-15. The NRA says the weapon can be used not only for hunting, but for sports shooting and self-defense. 
 
 For the “PBS NewsHour,” I’m John Yang.
– The Orlando shooting has raised anew the issue of the powerful weapons used by gunmen in mass shootings. While police initially said Omar Mateen used an "AR-15-type assault rifle," they have since clarified that he used a Sig Sauer MCX rifle. A look at some of the coverage: The Washington Post looks at those two weapons and finds that they're similar in broad terms: "a highly portable, customizable, easy to operate and accurate rifle." But one big difference is in the mechanics of how the bullet is propelled and the next round readied. It's "direct impingement" vs. "piston gun," as the Post explains here. The Sig Sauer can use magazines common to other semiautomatics, "but otherwise has no major parts that interface with AR-15s in any way, shape or form," says the Bearing Arms blog. (The post bashes media ignorance on the subject.) The Economist digs into the controversy over use of the term "assault rifle," which gun-rights advocates say is inaccurate because a gun such as the AR-15 "shoots only one round per trigger-pull." Orlando station WFTV offers a primer on topics such as the difference between semi-automatic and automatic weapons. (The AR-15 and the Sig Sauer are in the first camp; the military's M16 is in the latter.) PBS NewsHour did a segment on the AR-15 here. A Philadelphia Daily News columnist explains how she bought an AR-15 in seven minutes. Two Huffington Post writers got it done in 38 minutes. A New York Daily News writer fires an AR-15 for himself. He's "mostly terrified" as a result. Though he's taking flak for saying he got a temporary case of PTSD. FiveThirtyEight looks into the complicated topic of how such weapons are covered under gun laws.