A portable motor driven air gun powered by a power source includes a motor that is coupled to a pinion which drives a rack connected to a piston. The piston compresses air in a chamber producing high-pressure air. When sufficient energy is stored within the air stream by the piston, a valve opens which releases the compressed air to push a projectile through a barrel. The pinion rotates until it comes to an interrupted thread surface, at which point the rack and pinion are returned to the starting position via a spring. The piston may be coupled to a bolt thru a lost motion device to facilitate positioning of the projectile for firing. The direction speed and operative modes of the gun may be controlled with an electric circuit. The power source may be rechargeable, allowing the air gun to be operated independent from either a wall outlet or a compressed air supply.