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There were a number of different .303 inch rimless cartridges designed in the period 1917 to 1938 which were all strictly experimental in nature and will be described in the section on experimental ammunition. One however was made in considerable quantity and so is described here.
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".
Ball
From packet labels it appears the round was titled "Cartridge S.A. ball .303 inch Rimless". It was never formally adopted so was not
shown in Lists of Changes.
Despite the title, the case was semi-rimless and 62mm long with a rim diameter of 13.5mm and a head diameter of 12.8mm. The overall length was 78.8mm the same as the .303 inch Mark VII. Although never formally adopted, cases are stamped with the Mark numeral "I". The cases were Berdan primed and some cases have three fire holes.
The headstamp is either "R/|\L 18 I" or "I R/|\L 1918".
The bullet is a normal Ball Mark VII seated deeply in the case and secured with neck crimps.
The propellant charge was nitro-cellulose and firing trials in 1920 give the muzzle velocity as 2,750 fps.
Packet label for .303 inch Rimless
Armour piercing
The armour piercing version of the .303 Rimless utilised the same case as the ball round and was similarly headstamped, but usually had a green primer annulus.
The bullet was a normal Mark VIIW loaded deeply into the case. The bullet was secured by three pin stabs at the neck.
No information is available on penetration or ballistics, but muzzle velocity can be assumed to be similar to the ball round.
Dummy
Specimens exist of a dummy round in this calibre. These were almost certainly locally made at Woolwich or Enfield for gun functioning tests.
The case is the normal .303 rimless with four holes drilled in the case and a wood distance piece inserted.
The bullet is a Ball Mark VII and is soldered into the case.
Tracer
In 1919 further trials took place which included rounds loaded with the .303 inch SPG Mark VIIG bullet. The cases had a red primer annulus.
Incendiary
Also in 1919 Buckingham bullets were tested in the .303 rimless case. It is believed that these had a blue primer annulus.
The results of these trials were very unsatisfactory with a large number of separated case, believed to be caused by the "set-up" of the bullet before the deep seated bullet had cleared the case mouth. This may have been the cause, but the srmour-piercing bullets which did not set-up also suffered separated cases.
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