Range Report: The MCX Rattler

SIG MCX Rattler with bump helmet in the background

What do you get when you take some of the finest firearm engineers in the industry and ask them to design the smallest AR-based close combat weapon imaginable? Stipulations include that the gun needs to be piston-driven for the ultimate in reliability and ruggedness, modular for maximized flexibility, and chambered in .300 BLK so it will run a sound suppressor well. The culmination of that ballistic quest is the SIG MCX Rattler. This thing just drips cool.

Pertinent Particulars

The SIG MCX is actually a broad family of weapons. Easily interchangeable barrels and forearms combined with SIG’s proprietary recoil system offer unprecedented versatility. The action contains the reciprocating bits within the confines of the receiver. This feature negates the need for a buffer tube and allows for a side-folding buttstock or collapsible pistol brace. The end result is positively Lilliputian.

gas system on the MCX Rattler
The gas system on the MCX Rattler is readily adjustable with either a bullet tip or a standard set of fingers.

The sliding Pistol Stabilizing Brace allows the MCX Rattler to transfer as a standard handgun without any NFA baggage. This PSB collapses into the receiver for storage or portage and extends when extra stability is needed. The PSB includes the obligatory rubber forearm interface and Velcro attachment.

The gas piston action is readily adjustable using either the tip of a cartridge or your fingers. This allows the action to be optimized for subsonic or supersonic ammo. The forearm has plenty of M-LOK slots. The magazine release is replicated on both sides of the gun, and there is plenty of rail space up top for optics.

Czech vz61 Skorpion in front of a SIG Sauer Rattler
You really don’t appreciate how tiny this gun is until you juxtapose it against something small for comparison. The SIG MCX Rattler is not all that much larger than a Czech vz61 Skorpion.

The pistol grip is a bit smaller than standard but still remains both functional and grippy. The entire gun weighs just a bit north of 5 pounds. The forearm and barrel assembly swap out readily. There have been rumors that a 5.56mm conversion is in the making, but to a certain degree this would defeat the purpose of the gun. What really makes the Rattler unique is its .300 BLK chambering.

The .300 BLK is essentially a .30-caliber bullet seated into a slightly modified 5.56mm casing. By adjusting powder charges and bullet weights, the round may be configured for either subsonic or supersonic performance. When running subsonic ammo through a sound suppressed MCX Rattler, the rig is just stupid quiet.

SIG calls itself the Complete Systems Provider, and this is not an overstatement. SIG makes optics, ammo, accessories, and apparel. Most anything you could conceivably want for your SIG rifle is available from the parent company. Their .30-caliber suppressor is sealed for durability and features welded construction along with the most evil-looking miniature steel spikes on the end.

SIG Sauer MCX Rattler with suppressor attached
Even with a superb SIG sound suppressor installed, the MCX Rattler is not terribly bulky. The .300 BLK chambering makes the suppressed gun surprisingly quiet.

In a pinch, the device would make a superb prod should your foe end up both unarmed and recalcitrant. SIG’s electronic sights will hold their own with the best in the business. These sights are nicely matched to the personality and comportment of the host weapons.

Trigger Time

The SIG MCX Rattler seems heavier than it is—given its diminutive dimensions. However, the gun is legitimately tiny and runs like any other M4-style rifle. The Rattler will feed from any standard M4 magazine or drum.

At close combat ranges, the Rattler is eminently controllable. The gun will collapse down into something that could conceivably fit into the center console of a pickup truck. However, the Rattler can also be ready to go in moments. Nothing runs faster.

With the suppressor in place, the SIG Rattler runs quickly and well indoors. In confined spaces, you may still need earplugs, but it will indeed help preserve both your hearing and your situational awareness should you have to use the gun for real. With the can removed, the Rattler will tuck into a briefcase.

Technical Specifications

SIG Sauer MCX Rattler
Caliber.300 BLK
Weight5.1 lbs
Length Collapsed19.3 inches
Length Extended23.6 inches
Barrel Length5.5 inches with a 1:5 twist
ActionAdjustable Gas Piston
Handguard5-inch M-LOK


If the 5.56mm forend/barrel assembly ever hits the streets, the resulting package would be too short to accept SIG’s 5.56mm suppressor. Really stubby 5.56mm barrels release so much chaos they will wreak great mischief upon most 5.56mm sound suppressors. A 5.5-inch barrel on a 5.56mm weapon would look cool but would in essence create a very expensive .22 rifle. The Rattler really is optimized for the .300 BLK.

SIG Sauer .300 BLK MCX Rattler

LoadGroup Size (inches)Velocity (feet per second)
SIG Sauer 220-gr FMJ Subsonic1.0                                890890
Gorilla Ammo 125-gr Matchking0.41,776
HPR 220-gr HPBT0.4902
Hornady 125-gr JHP0.31,719

Group size is best four of five shots measured center to center, fired from a simple rest at 25 meters, using the SIG Sauer Romeo3 red dot sight. Velocity is the average of three shots across a Caldwell Ballistic Chronograph oriented 10 feet from the muzzle.

The SIG Rattler is a special purpose tool, and it is not cheap. There are rumors floating about that certain Tier 1 Special Missions Units bought a few specifically for executive protection missions. Given the exquisite design and unimpeachable level of quality imbued throughout, this seems a reasonable choice. Reliable, versatile, and just nifty as can be, the SIG Rattler packs unprecedented levels of awesome into the tiniest of packages.

Are you a fan of AR pistols? How does the SIG Rattler rank against your favorites? Share your answers in the comment section.

The Mission of Cheaper Than Dirt!'s blog, The Shooter's Log, is to provide information—not opinions—to our customers and the shooting community. We want you, our readers, to be able to make informed decisions. The information provided here does not represent the views of Cheaper Than Dirt!

Comments (12)

  1. I got mcx rattler pcb last year and all accessories like red dot sight, magnifier, flash and leather,hunting camera also I am waiting ATF approval for is pretty cool, I went NRA indoor range multiple times, it is very accurate on gun for home defense, hunting and indoor range.

  2. I feel for you. I own an MCX SBR, with Suppressor in 300BO. Legal with Tax Stamps here in Nevada. Great rifle!!!

  3. I would love to own one. But I generally top out at $1k prices, $1.5k for rare exceptions. I’m still thinking about it.

  4. I purchased a CZ Scorpion when they came available but when the MPX was launched I had to get one. Very well made now I’m trying to figure out how to afford that beautiful “Rattler”. The thing that is really left out on the reviews is this is not a plastic weapon.

    1. The thing I left out is that the military is now in love with the”Ratlery” not only the special ops but from what I understand this is going to be the pilots survival weapon. I just have a hard time understanding why it took so long but the 300 is another question why not the 556. I definitely understand using the Rattler in the special ops especially when a surpressor is used that’s the perfect weapon.

    2. The barrel is too short to effectively launch 5.56. Most of the powder burn time will take place after the bullet has left the barrel.

  5. I fail to see the practicality of an ultra small firearm using a large cartridge like the 300 BLK, especially when you get into the higher velocity loads.

  6. I’m sure its a wonderful weapon, however after building an RF85, 10.5″ SBR, for roughly 10% of the cost of the “rattler” I believe I’ll stick with mine. Its plenty small enough, wicked accurate and runs quite fast due to the RF85. I believe I’m a little better equipped for a larger variety of ranges also. I mean if they’re THAT close, I’ll introduce them to my TP9SA.

    As always
    Carry on

Your email address will not be published. Required fields are marked *

Your discussions, feedback and comments are welcome here as long as they are relevant and insightful. Please be respectful of others. We reserve the right to edit as appropriate, delete profane, harassing, abusive and spam comments or posts, and block repeat offenders. All comments are held for moderation and will appear after approval.

Discover more from The Shooter's Log

Subscribe now to keep reading and get access to the full archive.

Continue reading