CMMG Mk4 .300BLK Banshee Upper Group Receiver 12.5in Tungsten 30B8A6F-TNG. With the 12.5 BANSHEE upper group you can build your next 300BLK AR with character like no other. Specifications for CMMG Mk4 .300BLK 12.5in Banshee Upper Group Receiver:...