During the Second Boer War in South Africa (1899-1902), British authorities were obliged to re-evaluate rifle and ammunition design and tactics after facing Boer sharpshooters and snipers armed with Mauser Model 1895 rifles firing 7×57mm Mauser rounds with withering effectiveness, easily outranging the .303 British Mark II cartridge as regards accurate long-range fire.

The .303 British Mark VII round introduced in 1910 to which the British eventually modernized, for instance, has a projectile weight of 174 gr (11.28 g) and a muzzle velocity of 2,441 ft/s (744 m/s), while the .276 has a projectile weight of 165 gr (10.69 g) and a muzzle velocity of 2,785 ft/s (849 m/s) and a trajectory vertex at 800 yd (732 m) of 5.23 ft (1.59 m) against 9 ft (2.74 m) for the .303 Mk VII round.

[3] The combination of a slimmer more aerodynamic projectile—reverse engineering the trajectory from the previous paragraph indicates a ballistic coefficient (G1 BC) of approximately 0.55—with a higher sectional density propelled at a higher muzzle velocity and designed to travel at faster velocities, gave the .276 Enfield better ballistics than the .303 British Mark VII.

Several developmental versions of ammunition were prepared beginning in 1910, before approximately 1,000 Pattern 1913 Enfield rifles with 600,000 rounds of RL18000C trials cartridges were distributed to troops in 1913.

The troop trial RL18000C cartridges had slightly thinner case walls than earlier RL1800 iterations to increase powder capacity and an annular groove around the primer to prevent "caps out".

The great majority of troop trial ammunition was manufactured by the Royal Laboratory with the headstamp simply "R^L", but it was also made by Kings Norton, Kynoch and Greenwood and Batley.

Although the troop trial round was never formally approved, it was referred to as "Cartridge S.A. Ball .276 inch Mark I" in some documents and on the packet labels.

[4] The troop trial rifles and ammunition were distributed to the musketry school at Hythe, to one squadron in each of three cavalry regiments, and to one company in each of eight infantry battalions.

Canada, on the other hand, had some experience with the similar .280 Ross cartridge and appears to have taken interest in potential use of the P13 rifles to arm newly mobilized troops.

The design was modified again as the M1917 Enfield rifle chambered for the .30-06 Springfield cartridge, of which, Remington, Winchester and Baldwin manufactured 2,193,429 for the United States.

[10] The lack of a suitable (cooler-burning) propellant led to the design in 1914 of Improved Military Rifle (IMR) tubular nitrocellulose smokeless powder # 15 as a replacement for cordite for the .276 Enfield cartridge.