There is no doubt that the new wave of semi-automatic pistols — based on rifle designs that make use of a pistol brace — are the hot items in the firearms world. These new pistols allow you to carry very compact arms that have some of the attributes of rifles without obtaining a Short Barrel Rifle Federal Tax Stamp. SIG Sauer has entered the fray with the .300 Blackout MCX Rattler Pivoting Contour Brace (PCB) Pistol, and it is an excellent variant on the theme.

About the SIG Sauer MCX Rattler PCB

Based on SIG’s existing MCX Virtus rifle, the .300 Blackout MCX Rattler PCB Pistol is equipped with a 5.5-inch Personal Defense Weapon (PDW) barrel and a side-folding Pivoting Contour Brace. This allows the Rattler to fold into an extremely compact 16-inch package for transport and storage. This is possible because the MCX system uses recoil springs mated to the bolt carrier group instead of the standard buffer tube assembly. Without a setup like this, side-folding stocks aren’t possible on AR-15s.

The MCX Rattler PCB is a gas piston design. A gas piston setup (rather than the original AR-15 direct-impingement gas system) is preferable for a weapon like this. It will keep the operating system much cleaner.

The barrel features a free-floating M-LOK handguard system. The handguard also allows access to the gas port for adjustment when needed. This is particularly important when affixing a suppressor. A full-length rail runs atop the receiver and most of the barrel for mounting optics. The pistol grip is designed for improved one-handed shooting control.

SIG Sauer MCX Rattler Specs

Operating System:    Gas piston
Caliber:                         .300 BLK
Overall Length:          23 inches
Overall Width:           2.8 inches
Height:                       8 inches
Barrel Length:           5.5 inches
Weight:                       86 ounces (5.1 pounds)
Twist Rate:                 1:5
Mag Type:                  AR-15

Operating the SIG Sauer Rattler

The operating controls are the same as those found on SIG’s full-sized ARs. The controls also feature SIG’s excellent ambidextrous magazine release and ambidextrous safety/selector along with an extended bolt release. The bolt release extends below the folded PCB.

SIG’s PCB is unique in that it pivots open into a “V” shape to allow access to the Velcro forearms straps. The soft rubber sides of the PCB make a comfortable cheek rest for firing from the cheek-supported position. This is because it puts the pistol in the same line as it would be when fired off the shoulder (without making illegal shoulder contact).

SIG furnished a ROMEO3XL red-dot rifle optic for my test. It turned out to be very well-suited to the Rattler. The compact — almost micro — ROMEO3XL features three MOA and six MOA dots, with multiple intensity settings for varying light conditions. The MOTAC (Motion Activated Illumination System) powers the ROMEO3XL up when it senses motion and powers it down when the sight is stationary. The ROMEO3XL is very clear and bright and comes with a rubberized, easy-to-remove cover to protect the glass.

Rattler at the Range

The MCX Rattler ships with one 30-round Magpul polymer magazine. I took three different SIG Elite .300 BLK loads to the range for testing: the 120-grain Solid Copper Short Barrel load rated at 1,897 feet per second from a 6.5-inch barrel, the 120-grain Solid Copper Elite Hunting Load rated at 2,250 feet per second from a full-length barrel and the 205-grain Tipped Hunting load rated at 1,000 feet per second.

The Rattler’s gas port came set from the factory fully opened. I started out shooting the 205-grain hunting load, which did not have enough oomph to cycle the bolt to chamber fresh rounds. It would fire and eject the empty case but fell short of feeding the next round. This fine load is likely at its best in a full-length AR or from a bolt gun.

There were no cycling problems from the other two rounds. I immediately noticed the .300 BLK recoiled more than the 5.56 fired from a pistol — 55-grain bullets vs. 120-grainers at about the same pressure equals greater recoil — but it was still controllable.

If I fired the Rattler from a benchrest rather than a cheek support, I would have obtained rifle-level accuracy. The bullets landed dead-on when fired at 50 feet with the ROMEO3XL sight. My shooting partner obtained the same results: groups in the 5-inch or so range.

The SIG Sauer MPX Rattler PCB is cool. The extra weight and bullet diameter of the .300 BLK vs. the 5.56 makes it a more effective round at close range. It is a big, solid handgun, but so is a 6-inch-barreled Smith & Wesson .44 Magnum. It is top tier as a survival or get-home gun.

Online pricing for the Rattler PCB is currently around $2,299. MSRP of the ROMEO3XL is $719.99.


SIG Sauer:

About Scott W. Wagner

Scott W. Wagner has been a law enforcement officer since 1980, working undercover in liquor and narcotics investigations and as a member, sniper and assistant team leader of a SWAT team. He currently works as a patrol sergeant. He is a police firearms instructor, certified to train revolver, semi-automatic pistol, shotgun, semi- and fully automatic patrol rifle, and submachine gun. Scott also works as a criminal justice professor and police academy commander.