.303 "Rimless" Lewis
Hi Stonewall,
This round is an experimental round, and despite being described as "Rimless" on the packaging, it was actually semi-rimmed. Development started in 1917 in the hope of producing a .303" round with better ballistics than the Service Mk. VII Ball round, but restricted to the same overall length so that it would fit in the existing magazine.
After several designs had been considered, manufacture of the chosen type started, and it was then subjected to trials in modified P.14 rifles, and some machine guns. The loadind most commonly found is ball, but some AP rounds were made, which should have green primer annuli. A few steel dummies were also made. Although by 1918 the Army requirement for this round had disappeared, the Air Service continued to play with it for a short while longer, presumably for use in their Lewis guns.
There is quite a bit about this round in the late Peter Labbett's book "British Small Arms Ammunition 1864-1938" .
Regards.
Roger.