A shorter and lighter version of the original MLE—the famous Rifle, Short, Magazine, Lee-Enfield, or SMLE (sometimes spoken as "Smelly", rather than S, M, L, E) was introduced on 1 January 1904. The barrel was now halfway in length between the original long rifle and the carbine, at 25.2 inches (640 mm). The SMLE's visual trademark was its blunt nose, with only the bayonet lug protruding a small fraction of an inch beyond the nosecap. The new rifle also incorporated a charger loading system, another innovation borrowed from the Mauser rifle; notably the charger system is different from the fixed "bridge" that would become the standard. The shorter length was controversial at the time: many Rifle Association members and gunsmiths were concerned that the shorter barrel would not be as accurate as the longer MLE barrels, that the recoil would be much greater, and the sighting radius would be too short.
The iconic Lee-Enfield rifle, the SMLE Mk III, was introduced on 26 January 1907, along with a Pattern 1907 (P'07) Sword Bayonet and featured a simplified rear sight arrangement and a fixed, rather than a bolt-head-mounted sliding, charger guide. The design of the handguards and the magazine were also improved, and the chamber was adapted to fire the new Mk VII High Velocity spitzer .303 ammunition. Many early model rifles, of Magazine Lee Enfield (MLE), Magazine Lee Metford (MLM), and SMLE type, were upgraded to the Mk III standard. These are designated Mk IV Cond., with various asterisks denoting subtypes. During the First World War, the standard SMLE Mk III was found to be too complicated to manufacture (an SMLE Mk III rifle cost the British Government £3/15/-) and demand was outstripping supply, so in late 1915 the Mk III* was introduced. This incorporated several changes, the most prominent of which were the deletion of the magazine cut-off and the long range volley sights. The windage adjustment capability of the rear sight was also dispensed with, and the cocking piece was changed from a round knob to a serrated slab. Rifles with some or all of these features present are found, as the changes were implemented at different times in different factories and as stocks of preexisting parts were used. The magazine cut-off was reinstated after the First World War ended and not entirely dispensed with until 1942. The inability of the principal manufacturers (RSAF Enfield, Birmingham Small Arms, and London Small Arms) to meet military production demands led to the development of the "peddled scheme", which contracted out the production of whole rifles and rifle components to several shell companies.
In 1926 the British Army changed their nomenclature. The SMLE became known as the Rifle No. 1 Mk III or III*, with the original MLE and LEC becoming obsolete along with the earlier SMLE models. Many Mk III and III* rifles were converted to .22LR calibre training rifles, and designated Rifle No. 2, of varying marks.
The SMLE design was fairly expensive to manufacture because of the many forging and machining operations required. In the 1920s several experiments were carried out to help with these problems, reducing the number of complex parts. The SMLE Mk V (later Rifle No. 1 Mk V), used a new receiver-mounted aperture sighting system, which moved the rear sight from its former position on the barrel. The increased gap resulted in an improved sighting radius, improving sighting accuracy, and the aperture improved speed of sighting (making it also known as a "battle sight"). The magazine cutoff was also reintroduced, and an additional band was added near the muzzle for additional strength during bayonet use. Unfortunately, this design was found to be even more complicated and expensive to manufacture than the Mk III and so was not developed or issued beyond a trial production of this rifle numbered approximately 20,000 units, produced between 1922 and 1924 at RSAF Enfield. The No. 1 Mk VI also introduced a heavier "floating barrel" that was independent of the forearm, allowing the barrel to expand and contract without contacting the forearm and changing the zero of the rifle. The receiver-mounted rear sights and magazine cutoff were also present. Production numbered 1025 units, produced between 1930 and 1933.