The CQBR (Close Quarters Battle Receiver), with its 262mm-long is the modern equivalent of iconic Commando variants, and the replacement for the M4A1 Carbine.
— Item Description
The M4 CQB is a Submachine Gun.
The M4 CQB is a Submachine Gun. It represents a significant upgrade over the default engineer weapon, offering a much higher damage per shot while maintaining a similar rate of fire, accuracy, and effective range. Like the aforementioned weapon, it has extremely low damage drop for an SMG. It is nevertheless outclassed by most other engineer weapons unlocked later in the game.