The Lee-Enfield was designed before World War I and originally was to be replaced, but then the great war broke out and there was no time to design a new rifle. The Lee-Enfield proved extremely popular and effective in combat, with the spring-loaded bolt design allowing for a trained soldier to fire the bolt-action rifle extremely rapidly, with the average soldier being able to fire upwards of 20-30 aimed shots per minute. The bolt design of the Lee-Enfield was designed to pop back after opening the bolt, to allow for faster loading. The magazine also allowed for an imposing ten-rounds of .303 British ammunition, rapidly fed by two five-round stripper clips. As many as 17 million SMLEs have been built since they were designed and some are still produced and used today all over the world.
A redesign of the Lee-Metford (adopted by the British Army in 1888), the Lee-Enfield superseded the earlier Martini-Henry, Martini-Enfield, and Lee-Metford rifles. It featured a ten-round box magazine which was loaded with the .303 British cartridge manually from the top, either one round at a time or by means of five-round chargers. The Lee-Enfield was the standard issue weapon to rifle companies of the British Army and other Commonwealth nations in both the First and Second World Wars (these Commonwealth nations included Australia, New Zealand, Canada and South Africa, among others). Although officially replaced in the UK with the L1A1 SLR in 1957, it remained in widespread British service until the early/mid-1960s and the 7.62 mm L42 sniper variant remained in service until the 1990s.
As a standard-issue infantry rifle, it is still found in service in the armed forces of some Commonwealth nations, notably with the Indian Police and Bangladesh Police, which makes it the longest-serving military bolt-action rifle still in official service. The Canadian Forces' Rangers Arctic reserve unit still use Enfield No.4 rifles as of 2012, with plans announced to replace the weapons sometime in 2014 or 2015. Total production of all Lee-Enfields is estimated at over 17 million rifles. The Lee-Enfield takes its name from the designer of the rifle's bolt system—James Paris Lee—and the factory in which it was designed—the Royal Small Arms Factory in Enfield.
In Australia, Canada, New Zealand, and Southern Africa the rifle became known simply as the "303". The Lee-Enfield rifle was derived from the earlier Lee-Metford, a mechanically similar black-powder rifle, which combined James Paris Lee's rear-locking bolt system with a barrel featuring rifling designed by William Ellis Metford. The Lee action cocked the striker on the closing stroke of the bolt, making the initial opening much faster and easier compared to the "cock on opening" (i.e., the firing pin cocks upon opening the bolt) of the Mauser Gewehr 98 design. The rear-mounted lugs place the bolt operating handle much closer to the operator, over the trigger, making it quicker to operate than traditional designs like the Mauser.The rifle was also equipped with a detachable sheet-steel, 10-round, double-column magazine, a very modern development in its day. Although the design of the weapon itself is similar to a Mosin-Nagant, the weapons are almost completely different in function. Originally, the concept of a detachable magazine was opposed in some British Army circles, as some feared that the private soldier might be likely to lose the magazine during field campaigns. Early models of the Lee-Metford and Lee-Enfield even used a short length of chain to secure the magazine to the rifle.
The fast-operating Lee bolt-action and 10-round magazine capacity enabled a well-trained rifleman to perform the "mad minute" firing 20 to 30 aimed rounds in 60 seconds, making the Lee-Enfield the fastest military bolt-action rifle of the day. The current world record for aimed bolt-action fire was set in 1914 by a musketry instructor in the British Army—Sergeant Instructor Snoxall—who placed 38 rounds into a 12-inch-wide (300 mm) target at 300 yards (270 m) in one minute. Some straight-pull bolt-action rifles were thought faster, but lacked the simplicity, reliability, and generous magazine capacity of the Lee-Enfield. Several First World War accounts tell of British troops repelling German attackers who subsequently reported that they had encountered machine guns, when in fact it was simply a group of well-trained riflemen armed with SMLE Mk III rifles.
The Lee-Enfield was adapted to fire the .303 British service cartridge, a rimmed, high-powered rifle round. Experiments with smokeless powder in the existing Lee-Metford cartridge seemed at first to be a simple upgrade, but the greater heat and pressure generated by the new smokeless powder wore away the shallow, rounded, Metford rifling after approximately 6000 rounds. Replacing this with a new square-shaped rifling system designed at the Royal Small Arms Factory (RSAF) Enfield solved the problem, and the Lee-Enfield was born.