The CMMG Banshee MK57 is a semi-automatic pistol chambered in 5.7x28mm, with an 8-inch barrel and a 20+1 round capacity. 
 

Key Specifications 

• Manufacturer: CMMG 
• Model: Banshee MK57 
• Action: Semi-Auto 
• Caliber: 5.7x28mm 
• Barrel Length: 8 inches 
• Capacity: 20+1 rounds 
• Weight: 5.20 lbs 
• Safety Feature: Ambidextrous Safety Lever (CMMG Zeroed) 
• Grips Material: Black Polymer 
• Sights Type: Optic Ready (no iron sights) 
• Overall Length: 23.70 inches 
• Stock: Synthetic Black 
   

Detailed Features 

 
The aluminum frame keeps weight at 5.20 lbs despite the 8-inch barrel. The ambidextrous safety works for left- and right-handed shooters. The receiver is optic-ready with no iron sights installed, so a red dot or magnified optic goes on before use. The 5.7x28mm cartridge produces low recoil and high velocity, which makes for fast and controllable follow-up shots. 20+1 capacity is substantial. 
 
The 5.7x28mm is a specialized cartridge -- high velocity and relatively flat trajectory, but less available and more expensive per round than 9mm. Factor that into your long-term cost calculation before committing.

The CMMG Banshee MK57 gets consistently positive marks for its compact build, the 20+1 capacity, and the 5.7x28mm caliber's low recoil and velocity. Owners frequently note how manageable the platform is for fast follow-up shots. Build quality gets high marks -- the aluminum construction feels solid and the rifle holds up to use.

Customization options through aftermarket accessories get positive mentions from buyers who want to personalize the setup.

The predictable criticisms are price -- it sits above competing pistol-caliber options -- and ammunition cost/availability. 5.7x28mm is not as easy to find or as cheap per round as 9mm, which affects total ownership cost. Some newer shooters note the optic-ready-only setup means purchasing a red dot before the rifle is fully functional, which adds to upfront cost. The transition from traditional handgun handling takes some adjustment.