This new mini rifle could replace the decades-old MP5 for Delta Force and SEAL Team 6

The company who won the contentious contest to build America’s next military handgun is throwing its hat in the ring to provide a potential replacement for a weapon used by the country’s most elite counterterrorism units since the 1970s.

In March, U.S. Special Operation Command posted a notice to industry to come up with a new so-called “personal defense weapon” that had nearly impossible specs to achieve. The weapon had to be no longer than 26 inches with the stock extended, had to collapse to less than 17 inches AND be able to fire from the collapsed configuration.

The MCX Rattler features a 5.5-inch PDW barrel and can fire from a collapsed configuration. (Photo from Sig Sauer)

And oh, the weapon had to be made to fire both .300 Blackout cartridges and 5.56 rounds.

These rifles would replace the MP5 variants in special operations stocks — 9mm submachine guns that are both aging and offer significantly less effective range than more modern calibers compatible with subgun-length barrels.

Well, Sig Sauer, makers of the Army’s new M17 and M18 handgun, stepped up to the MP5 replacement plate with its new MCX “Rattler.”

“We had groups coming to us and saying the situations [they] were being put into with 9mm subguns, the caliber is not appropriate,” Sig Sauer officials said during a live event releasing the Rattler to the public.

The Heckler & Koch MP5 submachine gun of U.S. Air Force Senior Airman Samuel Caines, assigned to the Supreme Allied Commander Europe Security Detachment, ejects a bullet casing at the Training Support Center Benelux 25-meter indoor range in Chièvres, Belgium, Oct. 22, 2015. (U.S. Army photo by Visual Information Specialist Pierre-Etienne Courtejoie)

They wanted, “an escape gun that is going to have the firepower that [they] need.”

Based roughly on Sig’s MCX design, the Rattler has a 5.5-inch barrel and with its folding stock collapsed, the entire gun is just 16 inches long.

And it can fire in that configuration.

“The PDW stock allows you to function the gun when it’s folded,” Sig officials told RECOIL magazine. “It is the shortest rifle that’s on the market today.”

The H&K MP5K is about 12.5 inches long, but stocks of this weapon are getting long in the teeth and its 9mm round doesn’t have much range. (Photo from Heckler & Koch)

In fact, the Rattler comes in at just 3.5 inches longer than the ultimate CQB weapon — the MP5K.

“We wanted to give these guys a gun in a subgun size but that had the firepower to shoot out to 200-plus yards and effectively do what they needed to do,” Sig said.

The Rattler can fire suppressed in the 300 BLK configuration, but Sig says the barrel is too short for operating 5.56 cartridges with a can.

The Rattler upper is swappable with any standard M4 or AR-15-style lower, checking the box for the SOCOM PDW request to have the gun be able to change caliber in less than three minutes.

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