This is the weapon used in the Orlando shooting

The gun used by the shooter Omar Mateen early Sunday morning at the Pulse nightclub in Orlando, Fla., was a Sig Sauer MCX rifle and not an AR-15, contrary to early reports.

The MCX was described by the Orlando Police Department as an AR-15-style rifle, but the Sig Sauer, released in 2015, has capabilities different from those of the more traditional AR-15. Although the MCX uses an AR-15-style magazine and ammunition, other similarities extend only to outward appearance and destructive capability.

According to the Sig Sauer website, the MCX is “built from the ground up to be silenced, light and short” and uses “a broad array of accessories, enabling you to build a complete weapon system for any scenario or environment.”

The MCX was built to fire such rounds as the .300 Blackout, 5.56 NATO and 7.62 x 39mm. The 5.56 NATO round, which is often compared to similar but not identical .223 ammunition, is primarily used by AR-15 rifles. This feature makes the MCX as quiet as a pistol but as deadly as a rifle, according to the Washington Post.

The modular gun can easily adjust to different calibers and barrel lengths, due to the MCX’s quick change-barrel system. Weighing just 6 pounds, the MCX is compact and has interchangeable, customizable features.

This weapon is categorized as a “piston gun” because when the rifle is fired, the excess gas is pushed into a piston. The piston expedites reloading the weapon and is an improvement on the springs used in the “direct impingement” style of the AR-15.

The piston-style firing and the MCX’s 30-round magazine capacity make the assault rifle particularly lethal.

The MCX was not the only weapon Mateen used — he was also carrying a Glock 17 9mm semiautomatic pistol that has a standard 17-round magazine capability.

It is unknown how many magazines of ammunition Mateen carried with him on Sunday morning, but survivors’ accounts and Snapchat videos recall a near-constant stream of bullets throughout the shooting.

Despite having been on a federal watch list, Mateen easily passed a background check and was able to purchase the Sig Sauer MCX rifle and the Glock 17 9mm semiautomatic pistol within a week. Florida has a three-day waiting period for handgun purchases. However, when buying firearms from federally licensed dealers, the purchaser must pass a background check.

Mateen, who worked as a security guard, had a concealed carry license in Florida.

The controversy over the weapons used during the Orlando nightclub shooting has reignited a national debate about gun control. In 2004, Congress allowed the 1994 assault weapons ban to lapse. Efforts to reinstate the ban, which would prevent the sale of weapons like the Sig Sauer MCX and the AR-15, were discussed on the Senate floor Wednesday during a nearly 15-hour filibuster led by Senate Democrats.

AR-15-style rifles have been used in at least 10 recent mass shootings, including the 2012 shootings at an elementary school in Newtown, Conn., and at a movie theater in Aurora, Colo. Eugene Stoner, who designed the first AR-15 in the late 1950s, did not intend for the military-grade weapon to be used by civilians, his family members told NBC News.

“Our father, Eugene Stoner, designed the AR-15 and subsequent M-16 as a military weapon to give our soldiers an advantage over the AK-47,” the Stoner family told NBC News on Wednesday. “He died long before any mass shootings occurred. But we do think he would have been horrified and sickened as anyone, if not more by these events.”
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