There is a fairly lengthy explanation of the developments in the history of the 'Enfield' in Pegler's book

A bit around the introduction on the ShTLE


SAC seemed to have become almost obsessed with creating the perfect
rifle, and towards the latter half of 1901 it seemed that the solution may
finally have been found. Some 1,055 ‘Shortened Enfield Modified Rifles’
had been made at Enfield and sent for troop trials. The new rifle
incorporated a host of detail changes: these included a fully enclosing top
hand-guard along the barrel and two patterns of improved rearsight, ‘A’
and ‘B’, of which the former, graduated from 200 yd to 2,000yd (183–
1,829m), was eventually selected. Distinctive winged protectors were
fitted behind the rearsight bed and the straight-blade foresight was
similarly equipped with the addition of a novel and highly distinctive onepiece
steel forend, which both protected the sight and provided a secure
boss and lug fitment for the bayonet. Most significant of all, the rifle was
reduced in length by no less than 5in (127mm) and its weight by 1lb 4oz
(0.56kg), making it only 4.5in (114mm) longer than the Metford and
Enfield cavalry carbines.

The quality of manufacture and detail of these rifles was exquisite: ivory
knobs were fitted to the rearsight slide and fine adjustment provided for
elevation and windage. A modified pattern was sent for testing in mid-
December 1902, incorporating no fewer than 72 alterations from the Long
Lee-Enfield that preceded it. It was accepted for service later that month
(published in the List of Changes, 23 December 1902) as ‘The Short,
Magazine Lee-Enfield Rifle Mk I’. It is perhaps worth noting that over the
years, the comma after ‘Short’ has often been abandoned and many firearms
enthusiasts now believe that the nomenclature refers to a shortened
magazine, whereas it simply refers to the reduced barrel length of the new
rifle. Many units participated in the trials of 1902, including line regiments,
cavalry, Royal Navy and marine units. Although there were some minor
complaints, one persistent problem arose – that of poor accuracy on some
rifles – the reason for which could not immediately be determined.
Speeding up the loading process was also regarded as a major issue,
and after 1902 the first charger-loading Lee-Enfields were produced, with
five-round clips supplied for faster charging. To enable these to function
properly, the rifles were modified with the addition of a charger guide
attached to the left side of the receiver and the retrofitting of a new bolthead
with a sliding charger guide; as a result the distinctive steel bolt dust
cover was now omitted. Wear in the bolt-head charger guides resulted in
a steel charger bridge being fitted above the receiver, and this was to
become a permanent feature on all future Enfield models. A lever safety
catch was fitted to the rear left receiver.

These changes were just the beginning, though, for attempts to find
and correct the shortcomings with accuracy continued unabated.
Experimentation in 1904 at Enfield with different barrel types showed
that a short lead from the chamber into the barrel, allied to the adoption
of a new form of Metford/Enfield rifling, improved matters greatly. The
rifling modification required increasing the rifling twist towards the
muzzle, a configuration known as progressive rifling. It enhanced
performance by speeding up the rate of spin on the bullet and helping
stabilize it, but did not entirely solve the matter. Eventually a bore with
parallel but deeper rifling was introduced, but the two types co-existed
until almost the end of World War I.
Another immediate problem in introducing a shorter barrel had been
a slight drop in muzzle velocity to below 2,000ft/sec (610m/sec), and the
barrel modifications helped to raise the performance back to an acceptable
2,200ft/sec (671m/sec). (A full performance table for the .303in cartridge
is given on page 25.)
As a result of the testing, endless reports and recommendations, the
sealed patterns for the SMLE Mk I and Mk II rifles were approved on
26 September 1906 and began to be introduced for service on 26 January
the following year as the ‘Rifle, Short, M.L.E. No. 1 Mark III’. As was
inevitably the case, the number of amendments and design modifications
to the previous Mk I and Mk II were legion and space requirements make
it impractical to reproduce a full list, but the rifle that was to become the
most famous and recognizable long-arm in Britishicon military service had at
last taken on its final form.