The zero deflection gunnery technique was practised, among others, by British ace Albert Ball using Lewis guns on Forster mounts – which largely eliminated the need for either complex gun sights or aiming-off by eye. An elevation of +19° combined with ballistic properties of .303 (7.7mm) Brownings and the Defiant's operational speed made 'line of sight' aiming – as practised by Luftwaffe pilots – a practical proposition. This technique (described more fully in the article Schräge Musik) seems to have been neither taught nor practised by the RAF. Despite being common knowledge among veteran First World War aircrew, featuring in Air Ministry requirements reflected in fighter designs such as the contemporaneous Gloster G9 twin-engine bomber-interceptor – armed with five 20mm cannon at +12° – virtually all losses of Bomber Command aircraft shot down by Luftwaffe night fighters using upward-firing were ascribed to flak until 1944.