sentences:  In the first years of the war the IJN preferred the Type 99 Mark 1, and it did not operationally use the Type 99 Mark 2 until 1942.Towards the end of the war it developed a preference for installing the Type 99 Mark 2, presumably to counter the improving performance and ruggedness of US combat aircraft.The Type 99 cannon suffered from relatively low muzzle velocity and rate of fire compared to other 20mm cannons but the trade off was an extremely light cannon that did not hinder flight performance of the IJN's fighter aircraft.