This left an intermediate altitude at which a small gun could not be used because the range to the aircraft was too far, and a larger gun could not be used because the targets were moving too fast. During World War II this niche had been covered by the Bofors 40mm and similar weapons, but against modern jet-powered aircraft these were effectively useless because they simply didn't have the speed and weight of fire to be effective against targets that would be within range for only seconds. This led to the need for a new gun to address this intermediate-range role, and the Army defined this to be a weapon able to defeat aircraft flying at 1,000 mph (1,600 km/h) at altitudes up to 20,000 feet (6 km).