Abstract: A gas gun for producing pressure pulses for measuring the liquid level in a well. A body is adapted to be connected to a well and includes first and second chambers which are interconnected by a passageway. Pressure pulses flowing between the chambers creates a pressure pulse for transmittal to a well which is measured by a microphone measuring pressure variations. A piston actuated valve opens and closes the passageway between the first and second chambers. A firing valve transmits the higher pressure in one of the chambers to the back of the piston for closing the valve, and after one of the chambers is pressurized, exhausts the pressure from the back of the pressurized piston for opening the valve and creating a pressure pulse between the chambers. A spring biases the piston actuated valve to the closed position.