limited to reed switches optical sensors and mechanical limit switches furthermore additional sensors could be attached to the pinion 36 which would allow the control circuit 3 to determine the piston 5 location by counting revolutions and processing the information as it relates to the lead or linear inch per revolution of the lead screw 4 such information could be useful for altering the speed of the piston 5 and or the release of the spool 7 by controlling the power to the motor 1 and solenoid 18 at or near the point at which the spool 7 shifts open the motor can be reversed to bring the piston back to the starting position the spool operation in the preferred embodiment is controlled by the solenoid 18 and or the ball detent 19 the spool 7 is maintained in the closed position by using either a mechanical retention means such as the spring and ball detent or electromotive retention means such as from a solenoid 18 or both the spool 7 release pressure can be easily adjusted by increasing or decreasing the retention force holding it in place this provides a simple effective method for changing the energy delivered to the projectile by increasing or decreasing the pressure at which the spool 7 shifts open in the preferred embodiment when the pressure in the cylinder 14 reaches the set pressure the ball detent 19 is forced out of the way.