Abstract: A barrel unit for a nail driving gun includes an elongated base plate and an elongated cover plate. The base plate has a top side formed with a longitudinal discharge groove, a pair of upwardly extending positioning lugs at a rear portion of the base plate, and a pair of upwardly extending hooking lugs at a front portion of the base plate. Each of the positioning lugs has a rearwardly inclining rear end surface. The cover plate is formed with a pair of first slots and a pair of second slots which permit extension of a respective one of the positioning lugs and the hooking lugs therethrough. Each of the first slots has a rearwardly inclining end wall surface which abuts against the rear end surface of the respective one of the positioning lugs when the cover plate is superimposed on the base plate.