SIG Sauer MCX Rattler .300 BLK AR Pistol, 5.5 in Threaded Barrel, Short-Stroke Piston, Midwest Industries M-LOK Handguard, Side-Folding Brace
This SIG Sauer MCX Rattler stands out with its compact 5.5-inch .300 BLK configuration and proven short-stroke gas piston system. It is cleanly configured with an aftermarket Midwest Industries free-float M-LOK aluminum handguard and a continuous 1913 Picatinny top rail. Ambidextrous controls, a side-folding polymer brace with QD socket, and a factory-installed three-prong flash hider round out a precise, purpose-built AR pistol. Made in the USA with an aluminum alloy receiver and matte black finish.
Condition
Overall Condition: Excellent condition, showing only faint signs of any prior use or handling.
Bore Overall Condition: Excellent - Clean, mirror finish, no signs of wear.
Bore Rifling: Excellent Rifling - Sharp lands and grooves.
Specific Condition Notes: Evidence of only light prior handling. Overall presentation is exceptionally clean.
What's Included
- 1x 30-round Magpul PMAG for .300 BLK (marked "PMAG 30 AR 300 B")
- SIG Sauer factory hard case
- SIG Sauer safety instruction manual
- Cable lock
- Midwest Industries free-float M-LOK aluminum handguard (installed)
The MCX Rattler platform uses a semi-automatic, short-stroke gas piston action tailored for .300 AAC Blackout in an ultra-compact 5.5-inch barrel format. The barrel is threaded and fitted with a three-prong flash hider. The system incorporates a multi-lug bolt head, a dust cover, and omits a forward assist by design.
Controls are fully ambidextrous, including the magazine release and safety selector, supporting efficient manipulation from either side. The pistol wears a side-folding polymer brace with an integrated QD sling socket and a matte black finish.
This example features an aftermarket Midwest Industries free-float M-LOK aluminum handguard that aligns with the receiver’s 1913 Picatinny top rail to create a continuous mounting surface. The free-float profile provides generous M-LOK slots for accessories.
The aluminum alloy receiver set carries a uniform matte black finish and factory markings: "SIG SAUER Inc., Newington, NH, USA." Overall configuration is streamlined and purpose-built, with clean lines and a compact footprint.
Chambered in .300 AAC Blackout (7.62x35mm), the pistol has a 30+1 capacity and ships with a dedicated .300 BLK Magpul PMAG. It arrives in a SIG Sauer factory hard case with the SIG safety manual and a cable lock.