The CMMG Resolute MK4 Upper is chambered in .300 AAC Blackout with a 16.1-inch barrel in black. The MK4 designation indicates CMMG's standard AR-15-pattern upper receiver designed around the .300 BLK cartridge, which runs both subsonic and supersonic loads reliably. That flexibility is the main draw for this caliber — quiet subsonic loads with a suppressor, or full-power supersonic for hunting or range use.