As Editor In Chief of The Firearm Blog – the best gun resource in the universe* – I’ve had to transition from an operational role to more of an administrative role when it comes to reviews. And because we have such a talented staff with a wide breadth of knowledge and experience, my new position is just as fulfilling. However, I don’t get as much time behind the barrel as I once did, and I am forced to stick to my passions. For me, those are silencers and ultra compact weapon platforms. Combining those two loves is the SIG Sauer MCX rifle and pistol platform – a system I believe is in many ways superior to the widely regarded AR-15.

* Says me, that’s who.

As good as SIG Sauer marketing and advertising is at getting the word out on new products and their applications, I wanted to put together a comprehensive user-level guide for MCX rifle and pistol functionality, maintenance, upgrades and field uses. There are a variety of MCX features that are relatively unknown to most shooters. And there are still other attributes that are already highlighted but could use a deeper explanation.

I have planned out a multi-part series that will walk you the basics of the SIG MCX rifle and pistol platform from soup to nuts. I’ll use easy to follow steps and descriptions that aren’t too technical so anyone (for example, people like me) can understand. If I get off track I trust our readers will let me know.

Let’s chart a course through our exploration of the SIg Sauer MCX Rifle and pistol platform and get started.

I. Ultimate Guide: The SIG Sauer MCX Rifle And Pistol Platform – An Overview (you are here)

A. Overview

B. Specifications

C. Parts and Nomenclature

D. Variants

E. Where to Buy

F. Resources

G. Looking Ahead – The MCX Ultimate Guide

II. Ultimate Guide: The SIG Sauer MCX Operation (pending)

III. Ultimate Guide: The SIG Sauer MCX Maintenance (pending)

IV. Ultimate Guide: The SIG Sauer MCX Upgrades And Modularity (pending)

V. Ultimate Guide: The SIG Sauer MCX Military And Law Enforcement Applications

VI. Ultimate Guide: The SIG MCX Versus AR-15 (pending)

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Ultimate Guide: The SIG Sauer MCX Rifle, Carbine And Pistol Platform

A. Overview: SIG Sauer MCX Rifle Platform

The SIG Sauer MCX debuted in its original form in 2015 and was updated to its current form (2019) as the MCX Virtus. Currently the MCX is offered in 5.56x45mm and 300BLK with SIG and some dealers and distributors teasing a 7.62×39 chambering destined for the commercial market. Some parts and accessories are not interchangeable between the original MCX line and the newer MCX Virtus, so make sure to check your configurations before making a purchasing decision.

Built specifically for the 300BLK and to be run suppressed, the MCX’s short stroke gas piston operation with two user-selectable gas settings allows the shooter to configure their platform for supersonic or subsonic ammunition in 300BLK or weaker rounds in 5.56mm.

B. Specifications: SIG Sauer MCX Rifle Platform

• OPERATING SYSTEM:
  • Short Stroke Gas Piston
• CALIBER:
  • 5.56 NATO
  • 300 Blk
  • 7.62×39 (not yet available as of 12/2019)
• OVERALL LENGTH (IN):
  • 25 in (635 mm) – 35.5 in (905 mm)
• OVERALL WIDTH:
  • 2.8 in (71 mm)
• HEIGHT: 8 in (203 mm)
• BARREL LENGTH:
  • 5.5 in (140 mm)  – MCX Rattler
  • 6.75 in (171 mm) – MCX Virtus PDW
  • 9 in (229 mm) – MCX Virtus Pistol
  • 11.5 in (292 mm) – MCX Virtus Patrol
  • 16 in (406 mm) – MCX Virtus Rifle
• WEIGHT (OZ):
  • 5.5 lb (2.5 kg) – 6.4 lb (2.9 kg)
• MAGAZINE TYPE:
  • AR-15

C. Nomenclature: SIG Sauer MCX Rifle Platform

One of the first questions that arise when a shooter starts looking at the MCX is “how is this rifle/carbine/pistol different from my AR-15”. The answer depends on where you are looking. Much of the lower receiver is derived from the AR platform: the magazine well accepts standard AR-15/M16 magazines, the magazine release and safety selector (albeit ambidextrous) are in the same location, the trigger group and pins are designed for AR-15 triggers, and grips are interchangeable. Note however, not all AR-15 triggers are compatible with the MCX.

Here’s a small plug for SIG’s Parts Picker: Using live exploded diagrams and a parts list, owners can easily order replacement parts. Be aware, however, that some parts listed may require a certified armorer for replacement. It is an awesome resource, but you should know your limitations.

We’ll touch on this topic more in the third installment of the series with user-level maintenance, replacements and upgrades versus MCX certified armorer service and gunsmith level modifications.

From the outside, the MCX looks a lot like most carbines on the market today.

• 1. Buttstock
• 2. QD Sling Cup
• 3. Forward Assist
• 4. Upper Receiver
• 5. M-LOK Handguard
• 6. Pivot Pin
• 7. Magazine
• 8. Magazine Release Button
• 9. Grip
• 10. Ambidextrous Selector
• 11. Takedown Pin
• 11a. Flash Hider
• 12. Barrel
• 13. Full Length Mil-Std 1913 Rail
• 14. Bolt Catch
• 15. Charging Handle
• 16. QD Sling Cup
• 17. Trigger
• 18. Ambidextrous Magazine Catch

However, moving towards the rear of the firearm, there is an obvious lack of a buffer tube, buffer and buffer spring that would typically be found on an AR-15. Instead, the SIG MCX uses recoil springs around guide rods that are tied into the bolt carrier to operate the action. An operating rod attached to the recoil spring assembly makes contact with the piston that lives inside the gas block to cycle the action  during the firing sequence. And of course the charging handle doesn’t need a gas tube/gas key cutout, so it is not interchangeable with the AR-15 system.

Sometimes it’s the little things in life that make the difference. The MCX utilizes two replaceable roll pins as a mating surface for the charging handle, preventing excessive wear on the upper receiver (Number 9 in the above diagram). And there are other smaller key details that are important that we’ll touch on later.

The MCX features a Firing Pin Lock not found on the AR-15 platform that prevents the firing pin from moving forward and creating a slam-fire situation with soft primers sometimes found in non-NATO spec ammo. You’ll also notice that the bolt carrier is half the size of a standard AR-15 bolt carrier, which means it’s about half the weight. One of the hallmarks of the MCX system is a limited amount of reciprocating mass, making for a controllable and light recoiling weapon.

Above you can see some parts that are unique to the MCX platform. The operating rod (30), the firing pin lock (27) and the recoil springs (23) and guide rods (24) help comprise the carriage assembly.

The barrel assembly is designed to be replaced/upgraded by the end user with the use of a T-27 Torx bit and a torque wrench. While many AR -15 owners successfully swap barrels on their guns, that level of modification should really be performed by an armorer or gunsmith to prevent issues with head spacing and improper installation. MCX barrels are head spaced at the factory at least three times, eliminating the need for go/no go gauges.

The MCX’s gas block is not meant to be replaced at even the armorer level. Barrels are shipped as a complete system with a gas block pinned into place and a piston and gas valve installed. Obviously, longer barrels and different calibers will require different gas port sizes and gas block locations. To make proper contact with the Op Rod in the carriage assembly, the length of pistons will vary. Take a look at the lower barrel and piston (2) in the diagram below in comparison to the barrel on top. The gas block is moved forward and the piston is longer for the 5.56mm as opposed to a gas block that is closer to the receiver and a shorter piston for the 300BLK barrel.

As of the time of this writing, there are no licensed aftermarket barrel manufacturers for the MCX. Barrel extensions are permanently installed on MCX barrels at the SIG Sauer factory to ensure proper head spacing.

Like barrels, free float handguards on an AR-15 really should be swapped out by an armorer: barrel nuts need to be properly torqued to ensure safety and accuracy. However, swapping handguards on the MCX is as easy as pushing out the front pivot pin and sliding out the whole unit. The pivot pin retains the handguard through the Handguard Link pictured below.

SIG MCX upper receiver groups can be used on most AR-15 lower receivers with the addition of a lower receiver adapter kit that is included with most MCX Virtus upper receiver packages. The kit includes a screw in buffer tube attachment that will allow for folding stocks since a buffer tube, buffer and buffer spring are not required.

Because of the standardized rail attachment, a variety of SIG and aftermarket stocks and pistol braces can be exchanged by the end user with a T-27 Torx bit. Folding, collapsing, fixed and even buffer tube style stocks are widely available.

 

D.  Variants: SIG Sauer MCX Rifle Platform

For the purposes of this guide, I am going to stick with listing on the MCX Virtus variants – the current version of SIG’s pistol/carbine/rifle.

SIG Sauer MCX Rattler

https://www.sigsauer.com/store/sig-mcx-rattler-sbr.html

• Available in pistol or short barreled rifle configurations
• 300BLK or 5.56mm
• Uses a different upper receiver and barrel/gas block location and handguards than other MCX firearms.

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SIG MCX Rattler Canebrake

https://www.sigsauer.com/store/sig-mcx-canebrake.html

• Pistol configuration
• 300BLK
• Uses a different upper receiver and barrel/gas block location and handguards than other MCX firearms
• Faux silencer can be substituted with real silencer

SIG MCX Virtus Tacops 

https://www.sigsauer.com/store/sig-mcx-virtus-tacops.html

• 6.75” Barrel
• 300BLK
• Faux silencer can be substituted with real silencer

SIG MCX Virtus Pistol

https://www.sigsauer.com/store/sig-mcx-virtus-pistol.html

• 9” or 11.5” barrel
• 300BLK or 5.56mm

SIG MCX Virtus Patrol

https://www.sigsauer.com/store/sig-mcx-virtus-patrol.html

• 16” barrel
• 300BLK or 5.56mm

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MCX Parts:

MCX Rattler 300BLK Replacement Barrel *

https://www.sigsauer.com/store/mcx-virtus-rattler-5-5-caliber-exchange-kit-300blk.html

* Requires MCX Rattler upper receiver and 300BLK handguard

MCX Rattler 5.56mm Replacement Barrel

https://www.sigsauer.com/store/mcx-rattler-5-5-replacement-barrel-5-56nato.html

* Requires Rattler upper receiver and 5.56mm Rattler handguard

MCX Virtus 300BLK Caliber Exchange Kit – 6.75”

https://www.sigsauer.com/store/mcx-virtus-rattler-6-75-caliber-exchange-kit-300blk.html

MCX Virtus 300BLK Caliber Exchange Kit – 9”

https://www.sigsauer.com/store/mcx-virtus-9-caliber-exchange-kit-300blk.html

MCX Virtus 5.56mm Caliber Exchange Kit – 11.5”

https://www.sigsauer.com/store/mcx-virtus-11-5-caliber-exchange-kit-5-56.html

MCX Virtus 5.56mm Caliber Exchange Kit – 16”

https://www.sigsauer.com/store/mcx-virtus-16-caliber-exchange-kit-5-56.html

MCX Rattler 300BLK Upper Receiver

https://www.sigsauer.com/store/mcx-rattler-upper-assembly-5-5-300blk.html

MCX Rattler 5.56mm Upper Receiver

https://www.sigsauer.com/store/mcx-rattler-upper-assembly-5-5-5-56.html

E. Where To Buy: SIG MCX Firearms and Parts:

• SIG MCX @ Brownells
• SIG MCX @ Rainer Arms
• SIG MCX @ Palmetto State Armory
• SIG MCX @ Sportsman’s Guide

F.  Links and Resources: SIG Sauer MCX Rifle Platform

• SIG Sauer Operators Manual – MCX
• SIG Sauer MCX Product Page
• SIG Sauer MCX Parts 
• SIG Sauer MCX Small Parts Shopper
• SIG Sauer MCX Armorer Certification Course
• AR15.com – SIG Rattler discussion thread
• AR15.com – SIG MCX Virtus discussion thread
• TFB Review: SIG Sauer MCX Rattler – 1 Year Later
• HANDS ON: SIG’s Latest Canebrake MCX Rattler
• TFB REVIEW: The New SIG Rattler Canebrake
• TFB Review: SIG Rattler CaneBrake – NFA Edition
• King Abdalla II of Jordan and his SIG MCX
  
• TFBTV: The SIG MCX Rattler Review ft. the SIG SRD 762Ti Silencer
• SOCOM to Evaluate SIG Sauer’s MCX Rattler

G. Looking Ahead: SIG Sauer MCX Rifle Platform

Knowing the parts, basic differences from the AR-15, available models, and the benefits of modularity is a great start to appreciating the MCX system. Next time we’ll go into successfully operating the firearm, followed by maintenance tips and a look at the SIG MCX Armorers Certification. In a few weeks we’ll discuss ammo selection and adding suppressors make to make shooting safer more enjoyable.

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