CMMG Banshee Mk57 300 Series 5.7×28 mm AR-Platform SBR, Midnight Bronze, 5.5" Threaded Barrel
The Banshee Mk57 300 Series is CMMG’s short-barreled take on the 5.7×28 mm platform, built around the company’s patented Radial-Delayed Blowback system and finished in a distinctive Midnight Bronze Cerakote. Compact at roughly 21 inches with the stock collapsed, this factory-registered SBR balances a 5.5-inch threaded barrel with a lightweight free-float handguard, keeping weight down while maintaining full AR-15 ergonomics. Compatibility with FN Five-seveN pistol magazines and an ambidextrous safety make this micro-sized rifle both practical and flexible for a variety of roles.
Condition
Overall Condition: Very good condition, showing only light, superficial signs of prior handling or use.
Bore Condition: Excellent - Clean, mirror finish, no signs of wear.
Bore Rifling: Excellent Rifling - Sharp lands and grooves.
Specific Condition Notes: There are a few faint handling marks on the exterior finish, with no notable wear on high-contact areas.
What’s Included
- 2 × 20-round FN Five-seveN-pattern steel magazines
The heart of this Banshee is CMMG’s Radial-Delayed Blowback action. By delaying bolt opening through a specially angled bolt head, the system keeps recoil impulse low while maintaining reliable extraction with the high-velocity 5.7×28 mm cartridge.
The 5.5-inch barrel is threaded 1/2×28 and ready for muzzle devices or a suppressor. A full-length Picatinny top rail, numbered T10–T24, pairs with CMMG’s EML4 free-float aluminum handguard that offers M-LOK slots at 3, 6, and 9 o’clock for mounting lights, lasers, or grips.
Both receivers are aluminum—billet 6061-T6 upper and forged 7075-T6 lower—finished in Midnight Bronze for a uniform, corrosion-resistant surface. An ambidextrous selector and standard magazine release keep controls intuitive and familiar to AR users.
The polymer six-position collapsible stock allows quick adjustment to shooter preference while maintaining the Banshee’s minimal overall length. Flip-up polymer sights ride on the rail so the rifle is range-ready out of the box, with plenty of room for optics.