Sig Sauer MCX Rattler PSB .300 AAC Blackout Pistol – 5.5" Gas-Piston, Adjustable Gas, Ambidextrous Controls
The MCX Rattler PSB packs Sig Sauer’s proven MCX engineering into an ultra-compact .300 AAC Blackout platform built for maneuverability. A 5.5-inch nitride-treated barrel, short-stroke gas-piston system, and two-position adjustable gas regulator let this pistol cycle reliably with or without a suppressor. Fully ambidextrous controls, a quick-change barrel system, and a three-position telescoping brace further distinguish this factory-built PDW-style firearm.
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.
Rifling Condition: Excellent – Sharp lands and grooves.
Specific Condition Notes: Minimal handling marks only; finish remains even and undiminished.
What’s Included
- Factory SIG Sauer cardboard box
- SIG MCX owner’s manual
- SIG Sauer circular logo decal
- 1 x 30-round translucent Lancer L5AWM polymer magazine
The heart of the Rattler is its short-stroke gas-piston action, designed to keep fouling out of the receiver and maintain reliability through high round counts. A two-position adjustable gas regulator lets the shooter fine-tune performance for suppressed or unsuppressed fire without tools.
Sig’s MCX architecture gives this pistol a rapid, user-serviceable barrel conversion system. The lower receiver is marked “MULTI-CALIBER,” allowing caliber swaps within the MCX family when paired with the appropriate upper and bolt assembly.
The 5.5-inch carbon-steel barrel is nitride treated for hardness and corrosion resistance, and is threaded 5/8-24 to accept common .30-caliber muzzle devices. A fast 1:5 RH twist rate stabilizes the wide range of .300 BLK bullet weights, including subsonic loads.
A free-float alloy M-LOK handguard keeps the profile slim while providing rigid accessory mounting. On top, a continuous Picatinny rail is machined into the upper for optics or sights. Every primary control—safety, magazine release, and bolt catch—is ambidextrous, making the Rattler equally intuitive for left- or right-handed shooters.
The steel/aluminum brace telescopes to three lengths, folding flush against the receiver when stored. Combined with the compact barrel, it keeps overall length to a minimum without sacrificing stability during fire.