The RAF also evaluated the .5 inch Vickers and Browning guns. The results were inconclusive; the Browning was more powerful but was longer and heavier. It was concluded that the .303 inch version of the Vickers was almost as effective as the HMGs against the light, unarmoured aircraft structures of the time and it was much lighter as well as faster-firing. The RAF accordingly decided not to proceed with a heavy machine gun, while noting that any widespread adoption of armour for military aircraft would force a re-think. By the mid-1930s, when the increasing performance and toughness of aircraft began to cast doubt on the future of rifle-calibre guns, the RAF opted for the greater destructive power of a 20 mm cannon, choosing the French Hispano HS 404. A few American .5 inch Browning M2 guns were used late in the Second World War in applications for which the Hispano would have been too big and heavy, but apart from this no heavy machine guns were used by the RAF.