SIG Sauer MCX Rattler .300 AAC Blackout AR Pistol, 5.5-Inch Barrel, Threaded 5/8x24, Matte Black, Folding Brace
This SIG Sauer MCX Rattler stands out as a compact, .300 AAC Blackout AR pistol built on the bufferless MCX architecture with a 5.5-inch barrel and a two-position adjustable gas regulator. It features ambidextrous controls, a side-folding polymer brace, and a full-length MIL-STD-1913 Picatinny top rail. Presented in excellent condition, it includes a 30-round Magpul PMAG GEN M3 magazine for 300 BLK and the original owner's manual.
Condition
Overall Condition: Excellent condition, showing only faint signs of any prior use or handling.
Bore Condition: Excellent - Clean, mirror finish, no signs of wear.
Bore Rifling: Excellent Rifling - Sharp lands and grooves.
Specific Condition Notes: There are only a few small handling marks from prior use. Please refer to the photos for close-up views of the surfaces, controls, and markings.
What's Included
- Owner's manual
- One Magpul PMAG GEN M3 polymer 30-round magazine for 300 BLK
Built on SIG's bufferless MCX platform, the Rattler enables functional side-folding operation with its polymer brace, maximizing portability without compromising control. The short-stroke gas piston system with a rotating bolt promotes reliable cycling, and the two-position adjustable gas regulator (with +/- markings) lets the user tune the rifle for varying loads or conditions.
The controls are fully ambidextrous, including the safety selector, bolt catch/release, and magazine release, and it uses an AR-style charging handle. The pistol grip is SIG-branded polymer, complementing the compact, purpose-driven layout of the firearm.
A full-length MIL-STD-1913 Picatinny top rail runs the receiver length for mounting accessories, while the short M-LOK handguard includes an integral front QD sling socket. The 5.5-inch barrel is threaded 5/8x24 for compatible muzzle devices.
Markings include "RATTLER" on the model plate, "MULTI-CAL" on the ejection-port cover, and "SIG MCX MULTI-CALIBER" on the lower. The receiver is finished in matte black, and the firearm is marked "SIG SAUER INC., NEWINGTON NH USA." Chambered in .300 AAC Blackout (7.62x35mm), it carries a 30+1 capacity and ships with a 30-round Magpul PMAG GEN M3 magazine designed for 300 BLK.