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The SIG Sauer MCX Rattler PSB (Pistol Stabilizing Brace) chambered in .300 BLK is not just another compact AR platform. The adjustable pistol brace and diminutive size/firepower ratio offered are big indicators of this. The original MCX was spawned from a SOCOM request to develop a lightweight, compact rifle that was intended to be operated suppressed. Based on this, it is not surprising that the MCX’s initial chambering was the .300 BLK, with the 5.56mm quickly following. The Rattler is a specialized miniature offshoot (pun intended) of the MCX Virtus series of rifles.


The MCX Rattler’s operating controls and basic ergonomics are similar to the ever-so-prevalent AR-15—an important consideration for training/orientation purposes. The AR-like ambidextrous magazine and bolt release, along with a centrally-located, non-reciprocating charging handle, will be instantly familiar to any AR user.

The SIG Sauer MCX Rattler design concept originated with a request
from U.S. Special Operations units.

The MCX Rattler uses an AR-type fire control system and magazines. However, the bolt carrier group is different. This stemmed from the initial user’s request to use a folding stock in lieu of a typical AR buffer tube setup. SIG started with a gas piston system and then installed dual recoil springs above the bolt carrier group.

“All MCX Rattler features are intended for maximizing effectiveness during a fight. A return to basics, such as great handling with potent firepower, is often the answer. SIG Sauer offers such a platform with its MCX Rattler.”

Due to recoil spring placement, the charging handle sits slightly higher on the MCX than on an AR. Not only do the dual recoil springs eliminate the need for buffer tube, they also serve to smooth out/soften recoil impluse.  SIG Sauer has seen fit to produce select-fire, LE-/military-only Rattlers, as well as semiauto-only SBR and pistol versions for civilian consumption. With that said, while this article will focus on the Rattler pistol, I have also benefitted from experience with select-fire and semiauto-only SBR Rattlers—thanks to several days spent at the SIG Sauer Academy.

What sets the Rattler apart, even from its Virtus brethren, is its compact size, made possible via the 5.5-inch barrel. The 23.5-inch, sub-6-pound Rattler is chambered in .300 BLK. A free-floating M-LOK handguard is used. The Rattler does not offer barrel interchangeability that is typical of the other MCX variants. The Rattler was designed from the ground up to be as discrete as possible while retaining all the capabilities of the MCX.

The SIG Sauer ROMEO4T red-dot and SIG JULIET magnifier proved
ideal companions for the MCX Rattler.

ARs featuring barrels fewer than 10 inches are notoriously finicky, especially if considering suppressed use. Plus, do not forget that the AR buffer tube limits just how small ARs can realistically be shortened. Much of the SIG Sauer MCX Rattler’s development focused on finding proper placement of the gas port to optimize reliability, both suppressed and supersonic, as well as how best to stabilize the .300 BLK from the 5.5-inch barrel. This was how the 1:5 rifling twist was arrived at.

The MCX Rattler features an adjustable gas regulator, with the first position for normal operation; the second position is for when using a sound suppressor. Barrels for the Rattler are hammer forged and receive a nitride treatment for increased barrel life. It utilizes a self-regulating short-stroke piston combined with a fully locked-and-closed rotating bolt system that prevents any chance of out-of-battery failure/malfunction.

The Rattler’s adjustable brace is another feature that enhances its utility. The ability to compress the stock shortens the Rattler pistol to fewer than 18 inches. This is no gimmick; it is a real advantage to have rifle-like firepower in a package smaller than most SMGs.


A trend that can no longer be ignored or dismissed as a passing fad is the number of pistol models stemming from what was typically associated with rifle-pattern weapons via removing the buttstock. AR-type pistols are dominating this genre via weapons featuring shorter-than-16-inch barrels sans any rifle stock; only the buffer tube is protruding from the rear of the receiver. The AR buffer tube soon proved an advantage on these large pistols for numerous reasons. The buffer tube appendage was adapted to accommodate various types of braces to aid in supporting large, non-typical handgun forms.

The .300 BLK cartridge is well-supported by the industry via
numerous load options.

The MCX Rattler PSB features an adjustable brace, which allows for an even more compact profile, especially with the Rattler not requiring a buffer tube. For individuals who have no legal access to an NFA Class 3 short-barrel rifle (SBR), the brace-stabilized pistol converted long arm might well be the closest they can get to achieving rifle-like firepower in such compact packages. Along these same lines, the expansion of states issuing concealed-carry permits, along with honoring other states’ permits, is another reason large-format pistols are currently so popular.

The author, training at Echo Valley Training Center with the MCX Rattler pistol

Many will argue that there is no defensive/tactical value in such a nontraditional pistol—just use a full-size rifle or typical handgun. However, some will be lured to the braced Rattler pistol for the intriguing nuance of maintaining handgun status with a semblance of SBR rifle performance. The ability to be transported more discretely than a full-size rifle might translate into it being the weapon most readily available if a situation arises unexpectedly while away from home. The fact that a braced pistol is classified legally as a “handgun” offers users who have a concealed-carry license a certain amount of flexibility, compared to a true rifle or NFA SBR-classified weapon.

“The compact signature of the SIG MCX Rattler allows for easy transport in a smaller-than-expected package.”


We love to label things in the gun industry. The Rattler resists easy categorization. “PDW” (personal-defense weapon) is what most readily comes to mind, thanks to its compact size. One must be careful here. PDW terminology arose from compact weapons centered on new cartridge designs that facilitated the penetration of body armor out to a couple of hundred yards. The FN 5.7mm and H&K 4.6mm are prominent examples of these new cartridge types. PDWs were created during a time that NATO had images of body-armored Soviet Spetsnaz troops running amok in rear echelon areas. Times have changed along with the most likely threats faced. PDW weapons relied on their specialized cartridges to be effective, in terms of platform size and terminal performance. The SIG Sauer MCX Rattler does away with the earlier PDW specialized caliber limitations and trumps any handgun caliber. The SIG Rattler’s .300 BLK chambering allows for a multitude of missions. It instills more confidence than any handgun round or .22-caliber (or smaller) bullet typical of most PDWs when having to put down an opponent in close quarters, at distance or behind a barrier (such as a vehicle).

What distinguishes the .300 BLK is its ability to access standard supersonic rounds superior to 5.56mm for longer-range engagements while maintaining the trump card of subsonic ammunition for more-discrete, specialized work. Similar to the SIG MCX, the .300 BLK was created in response to a U.S. Special Operation military force’s request. One goal with the .300 BLK is the ability to launch .30-caliber projectiles from existing 5.56mm AR magazines without a reduction in magazine capacity or reliable functioning.

Anecdotal information compares standard-velocity .300 BLK 115- to 125-grain ammunition as matching the ballistics of the 7.62x39mm AK and eclipses 5.56mm—both in ballistics and terminal punch. The 5.5-inch Rattler barrel produced velocity in the 1,800 fps range with supersonic, 120-grain loads and approximately 840 ft/lbs of energy. This is a 40 percent increase in ft/lbs of energy over a similar-length barrel firing a 5.56mm 55-grain load.


The Eberlestock Big Trick pack serves as an excellent example of Eberlestock’s simple, practical ingenuity. Eberlestock gear is in service all over the world with Special Forces, regular military and police units. Eberlestock’s “secret,” as typified by the Big Trick, is to design a pack so that stuff is easy to find, along with making it easy to attach things to the outside of it. “Camouflage” is the use of any combination of materials or coloration for concealment, either by making objects hard to see or by disguising them as something else; the Eberlestock Big Trick bag does this.

Its combination of being a top-loader and featuring an accessible front panel means that the main compartment is hassle free to access. There’s no digging and wrestling with your pack to free an item—it is quickly at hand. With the Eberlestock Big Trick, there is much more than meets the eye. It is not just another pack. Much thought has gone into its design and construction.

Two rare-earth magnets conceal a hidden opening on each side of the bag, allowing ambidextrous quick access to a handgun concealed-carry pouch. The Big Trick features an expandable bottom that not only increases the pack’s overall volume, it also lengthens the scabbard, allowing you to customize the pack for load size and weapon length. These good-looking packs are perfect for going to the office or when traveling. Events around the world and here, in the United States, indicate that terrorists are committed to attacking civilian targets. A weapon other than a handgun would be much appreciated if one were trapped in such a nightmarish scenario. This is a very important situation that Eberlestock packs are designed to help solve. The convenience of easy adaptability for carrying multiple weapons, as well as ancillary gear (ammunition, optics, suppressors), is much appreciated.




It is only natural that a groundbreaking weapon such as the MCX Rattler be held to a higher standard. This stems not only from skepticism of how something can be made so compact and still maintain reliability, but also from the cost associated with the Rattler, compared to that of other pistols—i.e., AR or AK—stemming from rifle designs. With that said, if the Rattler makes good on the firepower potential offered and establishes a track record for rugged reliability, end users will not be able to resist.

The adjustable pistol brace complements the SIG MCX Rattler.

At the SIG Academy, multiple MCX Rattlers were handed out, including select-fire models. It would be fair to say that among the seven of us present, nearly 4,000 rounds of SIG Sauer 220- and 125-grain .300 BLK ammunition were fired from the MCX Rattlers over the course of two days. After using multiple Rattlers at the SIG Academy, I requested that a pistol version be shipped to me for more-thorough testing.

Multiple sessions at Echo Valley Training Center—involving firing from/around vehicles, the Jungle Walk Range and the Hesco Shoot House—instilled even more confidence in the SIG Rattler. A SIG ROMEO4T and SIG JULIET  4x magnifier were installed. They proved the ideal combination to get the most out of the Rattler. The Juliet magnifier was on a tilt mount, which allowed it to be swung out of the way until a situation called for its use. A SIG 7.62 suppressor was also pressed into service to round out the experience. This was easily accomplished, thanks to the SIG Taper-Lok muzzle device installed on the Rattler.

The culmination of SIG Rattler testing and evaluation for this article was participation in a Green Ops two-day tactical carbine/handgun course. An evolution involving working around barricades—requiring multiple magazines changes while moving between various firing points against a multitude of designated targets—served as a solid test for the Rattler. Based on firing positions, switching between shoulders was necessary. I really appreciated the MCX Rattler’s ergonomics and ambidextrous controls. Rifle training culminated with a scenario involving quickly advancing on a target or moving side to side while waiting on the command to fire, which then required bringing your momentum under control, with sights and trigger worked properly, to ensure hits.


Even after nearly two decades of attending firearms courses, the Green Ops experience rejuvenated my commitment to seek out training. Mike Green is the force behind the training received. His credentials—the ones he can report publicly—lend legitimacy to his methods. Green served in the Army, for which he was Special Forces and Ranger qualified (15 years). Mike taught thousands of students to shoot and trained hundreds of foreign Tier 1 units. More impressive is the list of instructor billets/qualifications he’s earned. Why? Just because you served in a Tier 1 unit does not mean you are a solid teacher and skilled conveyor of information.

We started on handgun basics, with Green Ops pointing out its own learned nuances. Mike’s intensive handgun training background, along with his gift for teaching, brought home the importance of grip, trigger control and sight alignment. Drills such as “Dot Torture” and “F.A.S.T.” helped establish skill level in the class. Rifle/carbine training commenced with a quick confirmation of zeros. Rifle training progressed in a similar fashion, as handgun with baseline drills served as a warm-up. Rifle reloading methods were covered and then segued quickly into transitioning from rifle to handgun. Steel was broken out for other drills fired at 50-plus yards that involved firing from different positions.

Some of the “nuggets” I took away from the Green Ops training experience involved giving myself a better base platform with my lower body, especially when running the rifle; not to overthink malfunction methods; and how to work better around barricades/obstacles with simple, yet brilliant, techniques. Green Ops’ practicality in approaching shooting and tactics was refreshing—in light of some of the instructors being “prisoners” of their own dogma or styles. This reflects real-world experience.




The MCX Rattler PSB lived up to the task of being a lightweight, fast-handling weapon. The piston-driven MCX proved utterly reliable, with only minimal effort given to wipe down the bolt carrier group and lubricate sporadically. The SIG ROMEO4T red-dot and SIG JULIET magnifier combination is worthy for consideration, because it enhances performance via the flexibility of offering pure red-dot or magnification. The .300 BLK SIG chambered MCX Rattler PSB’s raison d’être is as a platform just as at home dealing with tight-quarters threats or reaching out a few hundred yards. The increase in terrorism and acts of mass violence means that something more powerful than a handgun is a welcome addition to one’s personal armory. The compact stature of the SIG MCX Rattler allows for easy transport in a smaller-than-expected package.

SIG Sauer’s thought process with the MCX Rattler was to create a lightweight,
simple-to-operate weapon that was eminently adaptable to user needs. The
Rattler proves there is no need to turn to exotic PDW calibers.


SIG Sauer has become a true “one-stop shop” for military, law enforcement and civilian operators. Its Silencer Division was formally created in 2014, with its major introduction to the shooting public taking place at the 2015 SHOT Show. Through a revolutionary baffle design and unique manufacturing processes, SIG Sauer has eliminated the outer tube shell found in other rifle silencers. SIG Sauer utilizes 100 percent circumferential welds combined with wire electronic discharge machining (EDM) of the pathway of the projectile.

The SRD762-QD suppressor. Thanks to its quick-detach feature, it can move seamlessly between rifles, as well as being adaptable to multiple calibers up to .300 Win. Mag.

In addition to reducing weight, this tubeless design increases volume, thereby lowering pressure and  temperature for greater durability and increased signature reduction. For SIG’s rifle suppressors, a Taper-Lok Fast-Attach System was created, enabling quick and simple installation of a muzzle adapter device, thus eliminating the need for shims.

The SIG Sauer design optimizes alignment, resulting in improved accuracy and greatly reduced point-of-impact (POI) shift, even after multiple install/removal cycles. The Taper-Lok system allows users to access a variety of muzzle brakes and flash hiders for their rifles—all the while enabling SIG rifle suppressors to attach when needed. Basically, if the bullet diameter is .30-caliber or less, the SRD762-QD can be used. Of course, you should check with SIG Sauer if any “exotic” calibers or applications are being contemplated. The limiting factor is getting the Taper-Lok muzzle device mounted to your barrel.

Actual sound reduction differences between most respected suppressors are small. More important is durability, especially if high-volume firing is expected. Maintaining zero and accuracy when installed, removed and then reinstalled is paramount. Along these same lines, many will find that the flexibility of moving a suppressor between platforms is highly appreciated—while maintaining all of the above listed characteristics.

The SIG Sauer SRD762-QD rifle suppressor, mounted on the MCX Rattler.
The adjustable gas system is visible at the end of the Rattler’s forend.

“SIG Sauer’s thought process with the MCX Rattler was to create a compact, powerful weapon that was eminently adaptable to user needs.”

SIG Sauer’s thought process with the MCX Rattler was to create a compact, powerful weapon that was eminently adaptable to user needs. The decision to chamber the Rattler in .300 BLK is a nod to the reality that 5.56mm is no longer the only viable military-type rifle chambering. The ability to mount a suppressor on the Rattler’s 5.5-inch barrel while maintaining reliability is a bonus—not to mention access to widely available factory subsonic loads if desired.

A return to basics, such as light weight, great handling with potent firepower, is hard to
resist. SIG Sauer offers such a platform with its MCX Rattler.

All MCX Rattler features are intended for maximizing effectiveness during a fight. A return to basics, such as great handling with potent firepower, is often the answer. SIG Sauer offers such a platform with its MCX Rattler.



Avg. Velocity (fps) Avg. Group @100 yards (inches)

Best Group (inches)

Hornady 110-grain V-MAX




Hornady 208-grain A-MAX




Black Hills 125-grain Match




Black Hills 220-grain Match




SIG Sauer Elite Match 125-grain OTM




SIG Sauer Elite Match 220-grain OTM




NOTES: Accuracy test protocol consisted of three five-shot groups with each ammunition type at 100 yards. Group sizes were averaged. Velocity figures were obtained by averaging three rounds fired over an RCBS chronograph.



CALIBER: .300 Blackout

BARREL: 5.5 inches

OA LENGTH: 18 inches, brace collapsed; 23.5 inches, brace extended

WEIGHT: 5.25 pounds

SIGHTS: Flat top

ACTION: Short-stroke gas piston

CAPACITY: Detachable AR15/M16 magazines

MSRP: $2,719



Editor’s note: A version of this article first appeared in the August 2018 print issue of Gun World Magazine.