 An alternative was to fire "over" the propeller, especially in the case of a biplane aircraft, where the structure of the upper wing formed a convenient base.A drawback was that such a mount was less robust than a mounting on the forward fuselage, and the accuracy of the gun, especially at long range, was affected by vibration.The main difficulty, however, was the need for the pilot or gunner to have access to the breech of the gun, to change drums or belts as well as to clear jams.