The P40 was 'firing' gas guns. These are 'powered' by Propane gas, and utilise a rotary valve to allow a spaced series of ignition, creating the 'crackle' of gun fire, and muzzle flash. The giveaway is the 'rate of fire', and the lack of empty cases cascading from beneath the wings. The use of blank ammo would be extremely expensive, both in adding blank-firing barrels and ammo. Also, safety rules, especially at airshows, where this video is from, would never allow blanks to be fired, due to the hazard of the falling empty cases.