Both RAF fighters were armed with eight .303 Browning machine guns in the wings, harmonised by the squadrons to allow the bullets to converge at a distance. The Brownings had a high rate of fire and even a short burst from the eight machine guns sent out a large number of bullets. Although efficient against many aircraft, the small calibre bullets were often unable to penetrate the armour plating which was being increasingly used in "Luftwaffe" aircraft to protect crew and vital areas. An incendiary round, called the "De Wilde" was available, and this could do more damage than the standard "ball" rounds.