Sig Sauer MCX-Rattler LT Complete Upper Receiver 300blk 6.75″ UAMCX-300B-6B-LT-BLK

Sig MCX-Rattler LT Complete Upper 300blk 6.75″ UAMCX-300B-6B-LT-BLK Details
Nearly 10 years ago the MCX LVAW was introduced and became the choice of Special Forces groups worldwide.  With its 6.75” 300BLK design, the platform offered a replacement to sub-style platforms because of its ballistically advantageous caliber and a similar size footprint.

Now for the first time since its inception, the MCX-RATTLER LT upper assembly offers the same design that changed a generation of rifles with updated features that make it better than ever.

The MCX-RATTLER LT offers a 6.75” cold hammer forged barrel, compatibility with AR-style triggers, an adjustable gas system for suppressor optimization, a QD Clutchlok Muzzle Device and a M4 adapter kit to convert any M4 lower to utilize the folding capability of the MCX platform.  Also, the new MCX-RATTLER LT design allows it to be compatible with all MCX-SPEAR LT parts, meaning any barrel or handguard from the MCX-SPEAR LT can interchange parts, allowing for easy caliber and length conversions.  Advanced beyond any platform in its class, the MCX-RATTLER LT upper is the ultimate upgrade for your MCX or M4 lower.

Features:

Adapts with any M4/AR15 spec lower assembly
Dual rod recoil assembly eliminates buffer system
M4 adapter allows for pic rail folding stock capabilities
Adjustable gas system for suppressor optimization
Quick change barrel design allows to easily swap calibers and barrel lengths
Gas piston design reduces part exposure to carbon fouling
Designed to go 20k rounds before any part replacements
SLX/SLH Next Gen Suppressor designed QD Flash Hider for Clutch-Lok adaptability
Ambidextrous charging handle with gas deflector
Replaceable parts (cam path follower, feed ramp, forward assist/deflector, charging handle locking pins) allow for longer receiver life over standard AR designs.
Tapered bolt design functions more reliably and lasts longer than standard square lug M4 bolt designs.
Nitride coated cold hammer forged barrel for optimal accuracy and barrel life
Included items:

Complete 6.75″ 300BLK upper assembly includes our removable 3-prong flash-hider ready to accept our SLH & SLX suppressor systems.
Receiver extension delete stock adapter so you may mount any folding or static stock or brace that uses a vertical 1913 rail system.

Sig MCX-Rattler LT Complete Upper 300blk 6.75″ UAMCX-300B-6B-LT-BLK Specifications

The Sig MCX-Rattler LT Complete Upper 300BLK 6.75″ UAMCX-300B-6B-LT-BLK is a purpose-built, factory-engineered upper assembly designed for shooters who demand compact handling, reliable performance, and modern tactical versatility from their rifle platform. Chambered in .300 Blackout with a 6.75-inch barrel, this complete upper delivers one of the shortest, most maneuverable configurations available — ideal for close-quarters environments, suppressed shooting, and tight-space engagements.

Built on SIG Sauer’s SPEAR LT (Lightweight Tactical) operating system, the Rattler LT upper uses a robust piston-driven action that provides cleaner operation than traditional direct impingement systems. This results in reduced fouling and improved reliability, especially when using suppressors or extended firing sessions. Whether on the range or in duty applications, the piston design helps ensure consistent cycling and dependable performance.

The 6.75″ barrel length makes this upper exceptionally compact, giving shooters outstanding maneuverability in confined spaces — from vehicles to hallways — without sacrificing terminal performance when paired with .300 Blackout ammunition. The .300 Blackout chambering allows the use of both supersonic and subsonic loads, making the Rattler LT highly adaptable to suppressed builds and multi-role setups.

Finished in a durable black coating, the upper assembly offers corrosion resistance and long-lasting wear protection. The complete upper is ready to mount on any compatible MCX lower receiver, with precision machining that ensures tight tolerances and reliable fitment out of the box.

SIG’s lightweight handguard provides ample rail space for optics, lights, lasers, and accessories, making customization easy for the shooter’s mission profile. With its rugged construction, compact profile, and tactical capability, the Sig MCX-Rattler LT Complete Upper 300BLK 6.75″ UAMCX-300B-6B-LT-BLK is an excellent choice for PDW-style rifles, suppressed builds, and shooters seeking a high-performance close-range carbine platform.