SIG’s Rattler: SIG SAUER’s Piston-Driven PDW Extraordinaire

By Todd Burgreen | Photos by Richard King

The SIG SAUER MCX Rattler .300BLK is not just another compact AR platform. Its folding stock and diminutive size-firepower ratio are big indicators of this. The original MCX spawned from a United States Special Operations Command (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 .300BLK, with the 5.56mm quickly following. The Rattler is a specialized miniature offshoot—pun intended—of the MCX VIRTUS series of rifles.

Precisely as you would expect from SIG SAUER’s reputation for quality, the MCX design was not hastily introduced in knee-jerk fashion. In fact, SIG SAUER spent years honing the idea before introducing the MCX. The improvements in the MCX VIRTUS are evidence of even greater refinement. It is a distinct system with features that justify this claim.

The VIRTUS improvements include a tapered lug bolt group, two-stage SIG Matchlite Duo trigger, thicker receiver/barrel (the reason that MCX and MCX VIRTUS barrels/bolt groups are not compatible), M-LOK handguard and modified gas port location. All of the improvements were driven by additional Tier One DOD unit contract requests that sought to increase accuracy, modularity and durability. The MCX Rattler originated from a similar request.

SIG SAUER has seen fit to produce not only select-fire Rattlers exclusively for law enforcement (LE) and the military, but also semi-auto SBR and pistol versions for civilian consumption. With that said, this article benefits from experience with both select-fire and semi-auto SBR Rattlers thanks to days spent at the SIG SAUER Academy.

What sets the Rattler apart, even from its VIRTUS brethren, is the compact size made possible by the 5.5-inch barrel (1:5 twist). The 23.5-inch Rattler weighs under 6 pounds and is chambered in .300BLK, with a 5.56mm version surely arriving soon too. A free-floating M-LOK handguard is paired with a Rattler-specific MCX compact upper matched with a thin folding aluminum stock. For full disclosure, the Rattler does not offer the barrel interchangeability 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. SIG created the MCX Rattler SBR as the ideal choice for operators needing maximum firepower in a compact package.

ARs featuring barrels shorter than 10 inches are notoriously finicky, especially when a suppressor enters the equation. Yes, a few AR examples featuring 7-inch barrels do come to mind; however, these are the exception and do not offer the flexibility of .300BLK and 5.56mm chambering. Furthermore, do not forget that the AR buffer tube limits just how small ARs can be realistically shrunk. 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 stabilizing the .300BLK from the 5.5-inch barrel. This was how the 1:5 rifling twist came into being.

The MCX Rattler features an adjustable gas regulator, with the first position for normal operation and the second for suppressor use. Barrels for the SIG MCX Rattler are hammer forged and receive a nitride treatment to increase the barrel’s lifespan. The MCX 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 folding stock is another feature that enhances its utility, allowing the weapon to shrink to below 16 inches in overall length. This is no gimmick, but a real advantage in having rifle firepower in a package smaller than most SMGs.

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 facilitating 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 when NATO imagined body-armored Soviet Spetsnaz troops running amok in rear echelon areas. Times have changed, along with the most likely threats faced by the military and LE. PDWs 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 caliber limitations. The SIG Rattler’s .300BLK chambering allows for a multitude of missions to be satisfied, with the ease of suppression representing another trump card. The .300BLK instills more confidence than a 23-grain .22 caliber or smaller bullet typical of most PDWs when having to put down an opponent in close quarters or at a distance. The .300BLK’s raison d’être is as a CQB round for tight quarters and operational use indoors, where sound suppression is an important advantage—more on this later. The militarization of LE teams to deal with increasingly well-armed proponents of terrorism, organized crime and drug smuggling has given new purpose to the compact .300BLK rifle typified by the SIG SAUER MCX Rattler.

What distinguishes the .300BLK is the option of accessing standard supersonic rounds for longer-range engagements—to greater effect than with 5.56mm—while maintaining the trump card of subsonic ammunition for more discreet specialized work. Similar to the SIG MCX, the .300BLK was created in response to a request from U.S. special operations forces. The goal with the .300BLK was to launch .30 caliber projectiles from existing 5.56mm AR magazines without a reduction in reliability or magazine capacity. From anecdotal information, standard velocity .300BLK 115–125-grain ammunition equals the 7.62x39mm AK round in terms of ballistics and eclipses the 5.56mm round both in terms of ballistics and terminal punch. At 300 meters, the .300BLK has approximately 17 percent more energy than the 7.62x39mm. On exiting a 9-inch barrel, the .300BLK has the same energy as a 5.56mm M4 round leaving a 14.5-inch barrel, and it has greater energy than the 5.56mm as the range increases.

The MCX Rattler’s operating controls and basic ergonomics are similar to those of the ever-so-prevalent AR-15—an important consideration for training/orientation purposes. An AR-like ambidextrous magazine and bolt release along with centrally located non-reciprocating charging handle will be instantly familiar to any AR user. A flattop upper allows for any AR-type sighting system. 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 the typical AR buffer tube setup mentioned earlier. SIG installed dual recoil springs above the bolt carrier group. Due to recoil spring placement, the charging handle sits slightly higher on the MCX than on an AR. Not only did the dual recoil springs enable the folding stock, they also offered benefits in terms of a smoother/softer recoil impulse, eliminated the buffer tube wear associated with AR piston-driven rifles and increased overall reliability/durability.

SIG SAUER is blessed with the ability to place its products in the hands of writers or prospective clients and turn them loose at the SIG Academy under the watchful eyes of their world-class instructors. No need for a sales pitch. Simply get on the range and work the weapon during realistic training. As soon as the MCX Rattlers were broken out, other items were put aside. Such a unique, compact package with the firepower of a rifle had to be experienced. Multiple MCX Rattlers were handed out, including select-fire models. It would be fair to say that, between the seven of us at the SIG Academy, nearly 3,500 rounds of SIG SAUER 220-grain and 125-grain .300BLK ammunition were fired over the two days from four T&E MCX Rattlers.

While the Rattler’s name conjures up images of the poisonous snake, it might also hint at what the rifle will do to your teeth if not mounted properly during extended strings of fire. The minimalist nature of the SIG folding skeleton stock caused initial skepticism as to its ergonomics and effectiveness. This was quickly dispelled once the shooting started. The folding stock proved rock-solid and provided proper cheek weld for sighting with any optics; kudos to the SIG SAUER design team for this.

SIG SAUER ROMEO1 and ROMEO4T red dots were mounted on the Rattler’s Picatinny rail. The MCX Rattler lived up to the task of being a lightweight, fast-handling rifle. The piston-driven MCX proved utterly reliable, with only minimal effort required to wipe down the bolt carrier group and lubricate sporadically. For certain scenarios, a SIG SAUER JULIET 3X magnifier was mounted by the ROMEO4T. The JULIET magnifier was on a tilt mount, allowing for it to be swung out of the way until the situation called for its use. The combination of the SIG ROMEO red dot SIG JULIET magnifier is certainly worthy of consideration by LE, soldiers/Marines, civilians or private security contractors, as it offers users the flexibility to use a red dot with and without magnification.

The SIG Academy’s various ranges were used while handling the MCX Rattler, with the SIG instructors lending their input and creating drills. One evolution, for example, required multiple magazine changes while moving between various firing points in and around vehicles against a multitude of designated targets. Different firing positions necessitated switching between shoulders. The MCX Rattler’s ergonomics and ambidextrous controls were really appreciated. The culminating training evolution consisted of a “Jungle Run” course involving multiple MCX VIRTUS configurations to show off the weapon’s versatility. Long-range engagement and then CQB targets proved easy to handle with the MCX. An interesting twist at the end was the chance to finish the stage with the MCX Rattler in a “Hogan’s Alley” scenario.

SIG SAUER’s thought process with the MCX Rattler was to create a lightweight rifle that was simple to operate and immediately adaptable to user needs. The Rattler proves that there is no need to turn to exotic PDW calibers to achieve compact firepower. The flexibility to chamber the Rattler in .300BLK is a nod to realism, showing that 5.56mm is no longer the only viable assault rifle chambering. All MCX Rattler features are intended to maximize effectiveness during a fight or realistic training on the range. The key component in a fighting rifle is reliability. No matter how accurate, compact or powerfully chambered the rifle, if it does not work 100% of the time it is a liability. The selection of a fighting rifle is as personal as it gets. A return to assault rifle basics, such as reducing weight, improving handling and boosting the firepower, is often the answer. SIG SAUER offers such a platform with its MCX Rattler.



This article first appeared in Small Arms Review V22N4 (April 2018)
and was posted online on February 23, 2018


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