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303 SEMI RIMMED "LEWIS"

In 1918 there was a demand for a more powerful .303 inch cartridge from both the Army and the newly formed Royal Air Force. The Army's primary interest was for an armour piercing round to defeat German loop holes and sniper shields, whilst the Air Force wanted a more powerful round for observers' guns. The round developed was based on earlier design work in 1917 on a cartridge with a wider body than the Pattern '13 .276 inch and necked up to .303 inch. The resultant round was semi rimmed with the bullet seated deeply in the case to maintain the overall length of the Ball Mark VII.

It was intended that the Army would use the round in a rechambered Patten '14 rifle whilst the Air Force would use a modified Lewis gun. This gave rise to the cartridge being incorrectly named ".303 Lewis". (British Military Small Arms Ammo)