Shop › RRA CMPLT CAR A4 UPR 458 SOCOM 16" RRA CMPLT CAR A4 UPR 458 SOCOM 16" $790.00 Rock River Arms Tactical Carbine, Complete Upper Receiver, 458 Socom, 16" Barrel, RRA Operator Muzzle Brake, RRA 15" Free Float M-LOK Handguard, Anodized Finish, Black, Fits AR-15, Includes Bolt Carrier Group and Charging Handle Assembly SOC0564 Add To Cart Added! Rock River Arms Tactical Carbine, Complete Upper Receiver, 458 Socom, 16" Barrel, RRA Operator Muzzle Brake, RRA 15" Free Float M-LOK Handguard, Anodized Finish, Black, Fits AR-15, Includes Bolt Carrier Group and Charging Handle Assembly SOC0564