SIG Sauer MCX Rattler PCB

The Pocket-Sized .300BLK Nuclear Reactor
; .

The SIG SAUER MCX Rattler PCB is likely the most compact production AR-based
firearm in the world. Available now in .300BLK and 5.56mm, the Rattler PCB packs
hard-core firepower into a tidy concealable package.

“What do you need a new gun for?” Many is the gun nerd who has had his dreams mercilessly torpedoed by those fateful words. In fact, such a statement is unparalleled in the rarefied field of dream-crushing. However, in the case of the revolutionary SIG SAUER MCX Rattler PCB, there are some remarkably practical 21st century real-world applications for this most radical of black guns.

The Rattler PCB is quite literally the ultimate close-quarters defensive firearm. Tuck the Rattler behind the seat of your minivan and have no fear should you find yourself hemmed in by violent protesters bent on mayhem. Secure the gun against little fingers and rest it behind the door to your bedroom closet to be fully prepared for those times you hear breaking glass downstairs at 3 a.m. The SIG MCX Rattler PCB is an unabashedly fun gun with a practical side.


The charging handle is standard M4 sporting bilateral retention latches.

Visceral Impressions

I have written for the gun press for a quarter century now. Someday, if I keep plugging away at my word processor and going to Sunday School regularly, I hope eventually to join the ranks of such esteemed names as Huntington, Taffin, Venturino and Ayoob. To earn such rarefied company requires a literal lifetime of study and a reserved, sober view on the industry.

To show undue enthusiasm in the face of some greasy new smoke pole would be both unbefitting of my presumably expert stature and borderline unprofessional. I should indeed temper my zeal with patience and wisdom. A true professional gun writer should therefore approach any new concept gun with a stoic staid skepticism.

In this vein I transcribed my reaction verbatim when first I cracked open the cardboard after the UPS Brown Truck of Happiness delivered this remarkable product of the SIG SAUER Skunk Works — “Holy snot!”


If you really want to peg the funmeter — and scare “gun safety advocates” —
the SIG MCX Rattler PCB will also accept this 50-round skeletonized drum from X-Products.

SIG Pegs The Awesometer

The Rattler PCB is likely the smallest production rifle-caliber handgun in the world and chambered in .300BLK or 5.56mm. It is also available as a Short-Barreled Rifle (SBR) with SIG’s standard skeletonized aluminum side-folding stock. In this configuration the gun requires BATF registration and a $200 tax stamp.

The MCX Rattler PCB sports a two-position adjustable pistol brace, a 5.5″ barrel and a custom forearm for accessories. I previously felt the SIG MPX — a pistol-caliber large frame handgun packing enough cool to make Rosie O’Donnell look like Steve McQueen — was the absolute apogee of the gunmaking art. The MCX Rattler PCB raises the bar yet higher.


The SIG MCX Rattler PCB is a pistol in the eyes of the ATF. While the
Rattler PCB is slightly larger than most conventional handguns, it is an
order of magnitude more accurate, powerful and controllable.

What Does It Do?

The SIG MCX Rattler PCB crams everything righteous and wholesome about the AR15 into a concealable handgun-sized package packing legit .30-caliber firepower. The adjustable piston-driven gas system makes the gun reliable. SIG engineering makes it ergonomic. The gun’s chopped tube and abbreviated pistol grip even make it concealable. The synergistic combination makes it awesome.

The magazine release is bilateral, while the bolt catch is oversized and easily accessed. The gun is simply festooned with sling sockets. The Rattler PCB accepts all standard magazines while remaining thin, svelte and lightweight. The scary end comes adorned with an open-ended three-prong flash suppressor.

The pistol grip is a bit smaller than standard and textured for solid purchase. The ambidextrous charging handle is right where Colt put it in the first place, so the gun runs like your favorite AR, only smaller. The monolithic upper is seamless with the forearm while the forged lower includes a machined trigger guard.


The gas piston driven action is readily adjustable for subsonic and supersonic ammo.
Designed to accept the tip of a bullet, a standard set of fingers will do in a pinch.

Caliber Particulars

The tested .300BLK version of the SIG Rattler PCB feeds supersonic and subsonic rounds with comparable ease. The gas system adjusts using nothing more cumbersome than the tip of a cartridge or even a fingertip if you’re motivated. One setting is for supersonic ammo. The other is for the subsonic sort. We had no stoppages.

With a SIG SRD762 sound suppressor in place, the gun is laughably quiet with subsonic ammo. SIG cans are built like armored fighting vehicles. They craft the things out of Inconel 718, a nickel-alloy stainless steel liberally used on Navy submarines, jet engine thrust reversers and core components in nuclear reactors. The SRD762 is a radical and effective tubeless design that will outlive you.

The .300BLK cartridge has found a solid niche in both civilian and Law Enforcement applications. There are even a few spooky high-end military organizations purportedly with an operational affinity for the round. Jason Imhoff, the guy who thought up the .300BLK in the first place, is SIG’s Director of Ammunition Engineering. The true strength of the .300BLK cartridge rests in the fact it will run through standard magazines using standard 5.56mm bolt carrier groups while offering both subsonic and supersonic velocities dependent upon bullet weight.

Supersonic loads are entirely appropriate for hunting whitetail deer down here in the Deep South where I live. Subsonic rounds through a proper sound suppressor are just stupid quiet. With the Rattler PCB and a few cool-guy accessories from SIG, you get to pick your poison.


This is how the PCB is intended to be fired — keep in mind using the brace
as a shoulder stock could be interpreted by law enforcement as a violation of the law.

Behold the Revolution

The Rattler PCB sports an SB Tactical collapsible Pistol Stabilizing Brace (PSB). In case you’ve been living underneath a rock someplace, the PSB created an entirely new genre in defensive guns in America. Originally designed by a Physical Therapist to allow disabled veterans to run a gun one-handed, the PSB straps to the shooter’s forearm to provide enhanced safety and stability during one-handed operation. The addition of a PSB has also been found by ATF not to change the host gun’s classification from a pistol to a rifle. This means the weapon in question can legally retain its stubby pistol barrel.

The Rattler’s PSB has evolved since the gun’s initial introduction. The brace on my gun is an earlier version with a pair of steel bars that collapse into the mount in the manner of the M3 Grease Gun stock. The most recent Pivoting Contour Brace is minimalist in nature and folds discreetly to the side. All models include a rubber forearm pad with a length of Velcro material to secure it in place.


The SIG SAUER Romeo3 micro red dot is small enough for a conventional handgun.
It adds no appreciable weight and runs like a champ on the Rattler MCX PCB.

Combat Glass

SIG SAUER makes its own line of tactical optics. Their Romeo3 micro red dot is adequately compact to mount upon a handgun slide. When bolted atop the Rattler PCB, this little rig offers lightning-fast employment with no significant weight penalty.

The sight has a variety of light intensities to accommodate everything from bright daylight to hard dark. The whole mechanism is hardened against rough handling and feeds from standard CR2032 batteries available at any drug store. Waterproof, fogproof and sporting a 3MOA dot, the Romeo3 also comes with a skeletonized riser mount.


These are typical 15-meter groups fired from a simple rest. At handgun ranges,
the SIG MCX Rattler PCB pistol is rifle accurate. The SIG Rattler PCB could do this all day long.

Range Report

So what’s it really like to slip a SIG can onto the Rattler PCB’s snout and ring some steel? Holy crap. I could shoot this gun until I starved to death. The .300BLK round offers minimal recoil as well as hearing-safe acoustics in its subsonic guise. The Rattler PCB slithers around corners and within an automobile like literally nothing else. The PSB offers a stable firing platform eclipsing any conventional handgun. It also collapses small enough to hide in the center console of your pickup truck.

Even moving with a purpose, I could easily keep my rounds in the black at modest ranges. In fact, when firing from a rest and taking my time, I could produce single ragged holes all day long at 15 meters. Out to 100 meters and beyond the velocity, discrepancies between supersonic and subsonic rounds become significant. However, the SIG guys printed a 3″ group at 300 yards using supersonic ammo. Up close the Rattler PCB hits like a freight train, albeit a remarkably quiet freight train.

With the can in place, the gun is just front-heavy enough to mitigate any recoil or muzzle climb. The suppressor also keeps your fingers healthy, vibrant and attached to your hand. Without the suppressor you do need to mind your digits when moving quickly. This thing is tiny and it would be easy to let one wander.



Ours is a weird old world and getting weirder every day. Natural disasters, civil unrest, terrorist activity and good old-fashioned people being stupid inject the responsible armed American into some circumstances our forefathers might never have imagined. In such sordid environs as these, the Rattler PCB makes you the apex predator.

It’s hardly cheap, but the SIG Rattler MCX PCB brings rifle-grade accuracy to a pistol-sized platform. The gun is adequately powerful to reliably drop a whitetail but also rides comfortably within a modest knapsack. The Rattler MCX PCB is also just a hoot to shoot. In addition to all the practical stuff, the enjoyment it offers you on a lazy Saturday afternoon at the range is well worth the price of admission.

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