Short Magazine Lee Enfield. No. 4 mk. I
Right. We're going to go about this one by starting off with a little tangent.
In the latter part of the 19th century, the prevailing "wisdom" (if you can call it that) amongst the brass hats was that the infantryman should be equipped with a rifle with sufficient accuracy to hit a man at 1000 yards. These particular brass hats had cut their proverbial teeth during the muzzle loading era, so where on earth they got this idea from is a mystery. Also, where they got the idea that the average infantryman could even see his opponent, let alone be possessed of the skills to hit him, at 1000 yards is equally puzzling. Hence, military rifles of the period were very long. This is known commonly as the "Bisley school of thought".
This was not so good for the horsey types who couldn't handle the damn things on horseback, and gunners who had to carry their weapons while working around artillery pieces. These types were provided with shorter, carbine versions of the service arm.
Boer War experience with the MLM (Magazine Lee Metford), MLE (Magazine Lee Enfield) and their carbine versions showed that whilst the rifles had the accuracy, they were often badly sighted at the factory (these were the days before the soldier was expected to zero his own weapon, and indeed on these weapons the only adjustment was the range slide) so hits at extended ranges were more difficult. The cavalry and artillery discovered that their carbine versions were about as much use as a pork chop at a bar mitzvah in open warfare in the veld, especially against an enemy whose level of individual marksmanship was legendary, and who was equipped with a rifle and cartridge combination capable of excellent long-range accuracy. Plus, the PBI disliked lugging unnecessarily long rifles around the place.
In one of the extremely rare cases where a compromise works, and contrary to the prevailing school of thought in both Europe, Britain and the US, in 1903 the Rifle, Short, Magazine, Lee-Enfield Mk.I was adopted. It was intermediate in length between the MLE and LEC (Lee Enfield Carbine), and was capable of charger loading via a charger guide half on the receiver wall and half on the bolt head. Normally, in all walks of life, such a compromise would have satisfied nobody. This compromise, however, satisfied everybody: the PBI had a lighter, handier rifle still capable of the same practical level of accuracy, and the artillery/horseborne types now had a weapon capable of something more than mere Willy waving. In fact, the short rifle concept was immediately nicked by the Spams who hurriedly redesigned their new rifle as a short rifle (M1903 Springfield).
The rifle was updated again in 1907 to the SMLE Mk.III standard with the addition of a fixed charger bridge and a vastly improved tangent sight with windage adjustment. Aside from that, the rifle is fundamentally the same as the Mk.I, sharing the distinctive nose cap which helps protect the muzzle from being blocked by mud etc if pushed accidentally into the ground.
Combining the Lee bolt action with a fixed charger bridge was genius (belated, perhaps, but still genius), resulting in the fastest firing turn bolt action ever devised. The pre-First World War equivalent of the APWT required firing a minimum of 15 rounds in one minute, achieving hits with these rounds at 300 (verify?) yards. The record was obtained by Sgt Instr. Snoxall, who managed 38 rounds, all hits. For comparison, with the contemporary Mauser Gew. 98, 15 rounds in one minute is about the maximum that can be achieved.
Unfortunately, in the run-up to the First World War, the Bisley school of thought had come to dominance again, and the army was nearly shouldered with the .276" [Pattern 13 rifle], based on a Mauser bolt (but cocking on closing). This was certainly the most advanced rifle of its day, however it was not such a good battle rifle as the SMLE.
The standard of British marksmanship with the SMLE at the Battle of Mons in 1914 was so good that the Germans were convinced that they were being machine-gunned. There is even a report of a BEF company counter-attack using platoons fire and manoeuvre over 1000 yards of open ground that reached the German line with hardly a shot in response. When questioned, the Germans responded that this was because anyone who stuck his Pickelhaube up to try and get a shot in caught one in the head for his troubles.
In 1916/17, the SMLE Mk.III was simplified to the Mk.III*, with the elimination of the volley sights, Magazine cut-off and windage arm, largely because they were never used by the average soldier. In fact, the first two remarkably resemble the utility of tits on a fish. However, "hybrids" are often found with one or more "deleted" features.
Post-war, in the 1920s the [nomenclature] changed to Rifle, No.1 Mk.III, and during the 1920s troop trials were carried out with two experimental versions: No.1 Mk.V (extra band and aperture sights), and No.1 Mk.VI (which would, when simplified, become the [Rifle No.4]).
In addition, there were various .22 training models (which became various marks of Rifle No.2, but originally had "pattern 19xx" designations), some of which used caddy cartridges to permit charger loading, some of which were single shot, and some were magazine fed.
Production continued until the late 1940s in Australia, and until the late 1960s in India (7.62 mm NATO Rifle 2A and 2A1), and the SMLE was reportedly the weapon the Soviets most feared in Afghanistan, it far out-ranging their personal weapons (in perspective, an SMLE can kill things at half a mile while you're lucky to hit anything at 400 yards with an AK-47)