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The Sig Sauer MCX Rattler In .300 BLK: A Tough Son Of A Gun

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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.

SIG MCX Rattler with bump helmet in the background
The SIG MCX Rattler is as compact as physics allows an AR-based firearm to be.

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.

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

Caliber .300 BLK
Weight 5.1 lbs
Length Collapsed 19.3 inches
Length Extended 23.6 inches
Barrel Length 5.5 inches with a 1:5 twist
Action Adjustable Gas Piston
Handguard 5-inch M-LOK
MSRP $2,719


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

SIG Sauer 220-gr FMJ Subsonic 1.0                                890 890
Gorilla Ammo 125-gr Matchking 0.4 1,776
HPR 220-gr HPBT 0.4 902
Hornady 125-gr JHP 0.3 1,719

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.

Article posted with permission from Cheaper Than Dirt.  Article by Dave Dolbee.

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