 The initial Mark II, later known as the Mark IIA Series 1, went into squadron service in September 1940 at the peak of the Battle of Britain.Hawker had long experimented with improving the armament of the fighter by fitting cannons.Their first experiments used two 20 mm (.79 in) Oerlikon cannons in pods, one under each wing, (one aircraft was tested during 1940 with 151 Squadron) but the extra weight and drag seriously compromised the aircraft's performance and manoeuvreability, and the limited amount of ammunition carried coupled with the frequent stoppages suffered by the drum-fed guns, meant the arrangement was unsatisfactory.