By Dave Bahde

Top of the .300 BLK “short” list

The Rattler is fitted with an ultra-short 5.5-inch barrel, making it possibly the most compact/handy .300 BLK platform in existence. A three-prong flash hider is standard, but shown attached to the muzzle is SIG’s QD Taper Lock brake (also available in a flash hider) suppressor mount, allowing you to attach one of SIG’s own SRD762 QD suppressors to take full advantage of subsonic loads. The gas block is easily changed between suppressed and unsuppressed settings.

My introduction to the MCX Rattler occurred during a media event on the MCX Virtus. It was admittedly fun to shoot and it worked great using both super- and sub-sonic loads. Configured as an SBR, its stock made it as compact as anything chambered in 300 BLK ever used, and even with a 5.5-inch barrel and 1:5 twist it was plenty accurate. Capable of firing while folded added versatility no other AR style rifle could match without adding weight.

Available in both pistol (tested) with a collapsible brace, or SBR with a side-folding stock, like the rest of the MCX platform, the Rattler is fitted with fully ambidextrous controls. The handgrip is unique to the Rattler and features a more-vertical and slim profile. A monolithic Picatinny rail runs the distance across the top, and both sides of the lower-receiver and forend are machined for QD sling swivels.

As one might expect, using super-sonic ammunition it was loud with a tad more recoil. Sig Sauer’s SRD762-QD suppressor was already in house, making it easy to test the Rattler suppressed. Their SRD762 Taper-Lok Flash hider lets you attach the suppressor when needed and matches the factory tapered barrel. Called “Fast-Attach,” these suppressors are not really quick detach given the secondary latch. On the other hand, it does not come loose under fire, which was a primary request when they were developed.

Introduced in 2015, the Sig Sauer MCX is more rifle caliber MPX than AR. Using a short stroke piston system and abandoning the buffer tube allows for folding stocks, no stock or a pistol brace. Completely modular it can be configured to your needs. Operating controls are M4-like, with an ambidextrous safety and magazine release. If you were looking for the perfect platform for an M4 “like” PDW, the MCX Rattler is it. 

At the heart of the Rattler operating system is this dual-captured recoil spring/bolt-carrier-group assembly, which is not only largely responsible for the gun’s high-reliability and durability, but also the ability to run sans a buffer tube and be fitted with a side-folding or collapsible stock.

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