An gas supply system for use with an airsoft gun to power a pneumatic engine contained within a receiver of the airsoft gun. The gas supply system includes a bottle of pressurized gas that supplies pressurized gas at a first pressure to the receiver. A pressure regulator is located between the supply of pressurized gas and the pneumatic drive engine and operates to reduce the pressure of the gas from the first pressure to a second pressure. The pressure regulator is supported on the airsoft gun and allows the pressure of gas supplied to the pneumatic engine of the airsoft gun to be regulated. The pressure regulator includes an adjustment dial that allows an operator to change the pressure of gas supplied to the airsoft gun. The pressure regulator can be located between the bottle of pressurized gas and the receiver.