SIG MCX vs MPX: What’s the Difference, and Which Fits You?

The SIG MCX is a rifle-caliber family, tested here as the MCX Rattler in .300 Blackout with an adjustable short-stroke piston and standard M4 magazines. The MPX is SIG’s 9mm pistol-caliber gun, with a fixed piston gas system and proprietary 30-round magazines. Pick the MCX if you want more punch and the quietest subsonic setup, and the MPX if you want cheap 9mm practice and a famously smooth firing cycle.

	SIG MCX Rattler	SIG MPX
Caliber as tested	.300 Blackout (5.56mm barrels were in the pipeline in 2018)	9mm, convertible to .357 SIG and .40 S&W
Operating system	Short-stroke piston, two-position adjustable gas (subsonic and supersonic)	Gas piston, fixed gas system
Magazines	Standard M4 magazines	Proprietary 30-round translucent polymer
Barrel	5.5 inches	From 4.5 inches up
Muzzle threads	5/8×24	13.5×1 metric left-hand
Controls	AR manual of arms, bilateral safety and magazine release, no right-side bolt catch	AR manual of arms, bilateral safety, magazine release and bolt catch
Forward assist	None	None
Buffer tube	Not needed, recoil system in the receiver	Not needed, recoil system in the receiver
Brace as tested	Same sliding SIG pistol brace	Same sliding SIG pistol brace
Hand guard	Modular, swaps with an Allen wrench	Free-floated KeyMod hand guard

MCX vs MPX: Quick Answers

Which is better for home defense, the MCX or the MPX?

Pick your poison. Both guns take corners like a Bugatti and run easily indoors or inside a vehicle. The Rattler is a bit quieter with subsonic ammo and hits harder with supersonic loads. The MPX wins on cost, because it is tough to beat 9mm for cheap, high-volume training.

Do the MCX and MPX use the same magazines?

No. The MCX Rattler takes standard M4 magazines, which are cheap and easy to stockpile. The MPX uses proprietary polymer magazines that are a bit bulkier than MP5 mags but pack tidier, as long as you buy the matching pouches.

Can you put a 9mm suppressor on the MPX?

Yes, with an adapter. The MPX uses 13.5×1 left-hand threads on purpose, so nobody threads a .22-caliber muzzle device onto a 9mm barrel by mistake. Our author ran an inexpensive adapter to mount a SilencerCo Omega 45K. The Rattler is threaded 5/8×24 for .30-caliber cans.

Are these the current models?

These are the guns as tested in 2018. SIG has updated both lines since, so treat the table above as a guide to how the two designs differ, not a current spec sheet. Pistol brace rules have also changed since this review was written, so check current federal and state law before you buy a braced pistol.

For the newer MPX, read our SIG MPX new model review. For current picks, see our best 9mm carbines and PCCs, best 300 Blackout rifles and best PDWs guides.

Brothers in Arms: The Full Review

I have two younger brothers. While we are best of friends today, such was not always the case. In our earlier days, we strived vigorously to kill each other.

Brotherhood is an odd thing, and testosterone is the most potent poison known to man. The liberal application of this inimitable toxin within a familial environment can indeed result in some of the most deplorable mischiefs, but it can also be quite beautiful. Though brothers are inevitably competitive, obnoxious, loud, and violent, our bond is both unbreakable and complimentary. We might beat each other senseless with monotonous regularity, but perceive some collective threat from the outside and it’s on.

As I appreciate my two boys juxtaposed against my own fraternal experience I come to grasp that our many manifest differences make us a better whole. One might be stronger, while the other is a bit better at math. Independently we can excel in our own particular pursuits, yet together we can ably move a piano. Brothers bear uncanny similarities and profound differences. It is in such stuff as this we find our true strengths.

The Mission

I live half an hour outside of town on my modest rural farm. I count the local Sheriff and his deputies among my dearest friends, but even in a perfect world it will still take them at least a quarter hour to get to my place from a standing start.

I know of three of my immediate neighbors who have been the victims of home invasions in the past five years, two of which occurred while the homes were occupied. In one case the oldest family member present at the time was twelve. Defending yourself against an armed threat in your home is no baseless exercise.

Fortunately, there is an entire robust industry that has arisen around the guns, tactics, and gear required to keep you and your family safe come what may.

Whether the scenario is within your abode, in the local supermarket parking lot when you’ve zipped out late for a gallon of milk, or fleeing the coming zombie apocalypse, the flower of modern American engineering prowess offers you the tools you need to prevail. Amongst a cluttered sea of arms producers marketing their wares to the responsible armed American, nobody does it quite like SIG SAUER.

The SIG Success Story

In 2000 a pair of well-funded European investors purchased a group of five European and American defense companies that included SIGARMS, the immediate predecessor to today’s SIG SAUER. It’s breathtaking to contemplate, but this relatively modest subsidiary of the European SIG Empire sold for a paltry $1 at the time. Now some eighteen years later SIG is the apex predator among US tactical firearms manufacturers.

Not only does SIG produce roughly one million guns a year for consumption by Law Enforcement, military, and civilian end users, they also hold the contract to provide the US military with its new service handgun. With the award of the Modular Handgun System contract, SIG becomes only the third company in history to win a competitive contract for an American service pistol. It turns out that $1 outlay back nearly two decades ago was a remarkably prescient investment.

Nowadays SIG’s product offerings literally span the spectrum. From service pistols to concealed carry guns to 1911 variants in a bewildering array of configurations and hues, SIG’s handgun lineup includes something for literally everyone. Their long gun selection runs the gamut from 9mm to 7.62×51 with everything in between. Their product line includes optics, sound suppressors, air guns, ammunition, and gift items. Along the way, the good folks at SIG developed the most efficient home defense weapons available on the American civilian market.

Pursuing Perfection

What is the perfect home defense gun? That simple seven-word query has spilt a river of ink. Arguments have been made for the slide action 12-bore, various handguns, and Modern Sporting Rifles of countless stripes. However, all that pales when compared to the submachine gun.

SMG’s are pistol-caliber carbines designed to be fast handling and maneuverable. They were in their ascendancy during the global hemoclysm that was World War 2 and have steadily been replaced in LE and military service by stubby rifle-caliber weapons in more recent times. For civilian home defense use, however, literally nothing is better.

These little guns are safer and more easily mastered than a handgun, and their prodigious magazine capacity provides an insurance policy if the threat array arrives in herds. These guns typically accept modern electro-optical sighting solutions and will avail themselves of sound suppressors as well. Properly accessorized and wielded by a determined homeowner, such a smoke pole is indeed the top of the tactical heap.

Full auto fire is grossly overrated. Don’t get me wrong; I like producing a recreational brass rainbow just as much as the next guy. That thirty-round grin reliably creases my wizened visage despite a lifetime round count adequate to make the CEO of an ammo plant feel both warm and fuzzy. However, I’d much sooner be chased by some yahoo with a full auto MAC10 than by the same malevolent goober sporting a bolt-action hunting rifle with the will and skill to use it. The fact that our civilian defensive weapons lack a happy switch doesn’t make a lick of difference out here in the Real World.

Pertinent Particulars

Among SIG’s extensive product offerings, two guns stand out as particularly efficient home defense tools. Both weapons transfer as handguns yet sport collapsible Pistol Stabilizing Braces. Just in case you’ve been living underneath a rock lately, the PSB is an inspired contrivance devised by a Navy veteran named Alex Bosco to allow disabled veterans home from our recent kinetic festivities overseas to run a proper gun safely one-handed.

The BATF, showing remarkably sound judgment, ruled that the addition of one of these delightful devices did not change the host gun’s pistol classification. This ruling opened up a whole new classification in American firearms. As a result, short-barreled pistols sporting a PSB bring most of the efficiency of a subgun without all of the legal hassle.

Our Contestants: 9mm and .300 Blackout

The hottest of the lot within the SIG lineup are the MPX and the MCX Rattler. The MPX is a closed-bolt, pistol-caliber large-frame handgun that is convertible between 9mm, .357 SIG, and .40 S&W. The MPX uses proprietary 30-round translucent polymer magazines and sports the same controls and manual of arms as your favorite AR rifle.

The charging handle and last round bolt hold open are right where Gene Stoner put them in the first place. The magazine release, bolt catch, and safety are bilateral for easy access. Unusual for a pistol-caliber firearm, the MPX is actually a gas-operated piston-driven design and enjoys an unnaturally smooth firing cycle as a result. Barrels and buttstocks are easily exchanged in the field by the user. Barrel lengths range from a stubby 4.5 inches on up. The gun includes plenty of KeyMod space for accessories, and the barrel is threaded for a muzzle attachment.

The muzzle threads of the MPX are 13.5×1 metric left-handed threads. This precludes the addition of standard flash suppressors and muzzle attachments for good reason. As 9mm and 5.56mm muzzles are both typically both threaded 1/2×28 in America, the use of this thread pitch would allow the addition of a .22-caliber muzzle attachment to the muzzle of a 9mm MPX. Such an appendage would instantly convert your expensive home defense gun into a bomb. However, I landed an inexpensive adaptor online that sits between the MPX muzzle and my SilencerCo Omega 45K sound suppressor. Just don’t inadvertently thread on something with a .22-caliber hole in the end, and you’re good.

The SilencerCo Omega 45K is a sealed stellite and stainless steel can that is compact, efficient, and essentially indestructible. Interchangeable pistons allow for different applications. I use this .45-caliber can on 9mm firearms to great effect simply by exchanging the mounting pistons. The Omega is a superb sound suppressor that represents the current state of the art.

Now For Something Spunkier

The MCX Rattler is one of the shortest production rifle-caliber AR pistols ever made. The Rattler also sports a collapsible PSB. Based upon an otherwise-stock AR lower receiver, this component of the revolutionary Rattler is the only thing that is standard. The adjustable short-stroke piston-driven gas system is uber-reliable and easily manipulated with either a cartridge tip or a standard finger. The gun’s modular design allows it to be configured with various barrel lengths, buttstocks, and calibers in the field with nothing fancier than an Allen wrench.

While 5.56mm barrels are in the pipeline, the most common variant of the Rattler pushes .300BLK.300BLK allows the option of either stupid-quiet subsonic loads for maximum stealth or supersonic rounds for those times when you need a little more downrange horsepower. The 5.5-inch barrel makes the MCX Rattler incredibly compact. SIG engineering and the .300BLK chambering make it unnaturally versatile.

The muzzle of the MCX Rattler is threaded 5/8×24 for a .30-caliber sound suppressor. In addition to just about everything else, SIG makes these, too. Their SIG SRD762 is the top of the line.

The SRD762 is a novel tubeless welded can built from Inconel and sporting wrench flats on the front. There are also some truly scary-looking teeth should you feel the need to set the gun against a door facing or prod some recalcitrant miscreant. This can threads directly onto the gun’s muzzle and secures in place via a Taper-Lok shoulder for minimal POI (Point of Impact) shift. It weighs about a pound and runs like a champ.

Similarities

Both guns sport an essentially identical manual of arms. Each weapon employs the classic Stoner-designed T-handle for charging and offers similar controls. Each magazine well is flared slightly for fast reloads, and the guns both have plenty of rail space for accessories. Both designs also keep their recoil systems inside the receivers, negating the need for a buffer tube out the back.

They are modular designs that allow easily interchangeable forearms, buttstocks, and barrels. We gun nerds like to accessorize our modern tactical weapons more vigorously than our young daughters might individualize their Barbie collection. The MPX and MCX Rattler are comparably versatile in this regard. Neither gun includes forward assist device.

They both use the same remarkable sliding Pistol Stabilizing Brace. These contrivances mount a firm rubber arm brace on the end of a pair of steel struts akin to those of the M3 Grease Gun. A top-mounted pushbutton releases the device for extension or stowage. They are both comparably comfortable.

Differences

The MCX Rattler uses standard ubiquitous M4 magazines, while the MPX employs a proprietary translucent polymer version a bit bulkier than those of an MP5. M4 mags are cheaper and easier to stockpile, but the MPX boxes are smaller and tidier to pack, albeit in proprietary mag pouches. Both magazines include a substantial curve that helps you get them oriented in the dark.

The Rattler is a sub-variant of the superlative SIG MCX rifle series. This gun can be configured in literally hundreds of different combinations to address specific missions and circumstances. Unlike the MPX, it lacks a right-sided bolt catch. By contrast, the MPX is a unique small-framed design. The MPX gas system is fixed. That of the MCX Rattler is adjustable.

Impressions

Pick your poison. Both guns take corners like a Bugatti while running easily indoors or within a vehicle. The MCX Rattler is perhaps a bit quieter with subsonic ammo and hits harder with the spunky stuff. However, it’s tough to beat Herr Luger’s 9mm for cheap high-volume training.

Both weapons are unabashedly expensive, but you get what you pay for. Superlative ergonomics, perfect execution, and brand cred that are both hard-won and comparably justified come at a cost. However, if you want to run the apex predator as your primary home defense arm, one of these two brothers is it.

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