First Look: SIG Sauer MCX Rattler LT .300BLK Upper

A 6.5 inch barrel means it's easy to handle in tight spaces.

posted on May 18, 2024
SIG Sauer MCX Rattler LT .300BLK Uppe

SIG Sauer is now offering a new short-barrel upper-receiver assembly chambered in .300BLK for the MCX Rattler LT series of short-barrel rifles and large-format pistols. This drop-in upper works with any MCX Rattler LT lower receivers and offers users the chance to fire the popular .300BLK cartridge through their existing SIG Sauer MCX Rattler LT firearm with a shorter, 6.75-inch-long, suppressor-ready barrel. These upper receiver groups also make use of adjustable gas systems to adjust for their shorter barrels and to work efficiently with suppressors.   

SIG Sauer MCX Rattler LT Features:

  • Adapts with any M4/AR15 spec lower assembly
  • Dual-rod recoil assembly eliminates buffer system
  • M4 adapter allows for Picatinny rail folding stock
  • Adjustable gas system for suppressor optimization
  • Quick-change barrel design allows to easily swap calibers and barrel lengths
  • Gas piston reduces part exposure to carbon fouling
  • Built to fire 20,000 rounds before any part replacements
  • SLX/SLH Next Gen Suppressor 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 functions more reliably and lasts longer than standard square lug M4 bolt
  • Nitride coated cold hammer forged barrel for optimal accuracy and barrel life

The SIG Sauer MCX was introduced and became the choice of Special Forces groups worldwide. The platform offered a replacement to pistol-caliber platforms with superior ballistics and first-rate subsonic performance thanks to use of subsonic .300BLK ammunition. The MCX-Rattler LT upper assembly offers the same design that changed a generation of rifles with updated features that reflect the needs of today’s gun owner. To learn more about this new upper receiver or other products from SIG Sauer, visit


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