The AWM currently has a three month exhibition titled "Lawrence and the Light Horse". On display is an SMLE reportedly captured by the Turks at Gallipoli, then captured from the Turks by Arabs and presented by them to Lawrence who used it during the War and presented it to George V after the war. The Queen has loaned it to the AWM for the current exhibition. It has a Volley Sight and I thought the following from Mike Cecil, Head of Military Heraldry and Technology at the AWM might be of interest to members.
"Its a 'Tangential Sight' or 'Long Range Volley Sight 'or 'Long Range Dial Sight'. The 1888 Long Lee Enfield trials rifle paperwork simply calls it a 'Dial Sight', so this is the safest description. They were standard on all .303 rifles until 1915 (including the Long Lee Enfields).
There are two parts: the dial on the side of the timber work 1/2 way along, and a flip up aperture sight that is nestled in behind the safety catch on the left side of the action at the rear. It pivots on the safety catch pin and uses the same external spring to provide tension. When flipped up, the lugs engage notches in the spring and keep the aperture sight erected with a slight forward angle. The Dial Sight part is rotated to the desired (or estimated) distance, from 1700 yards to 2700 yards, and the rifleman sights through the aperture and across the top of the lug of the handle on the dial sight. The rifle is canted up at what seems to be an absurd angle from the line of sight, especially with the longer ranges. You actually sight along a tangent adjacent to the barrel line, rather than along the barrel or at a converging angle as you would with the normal adjustable battle sights (hence one of the descriptions as a 'Tangential Sight')
The Dial Sight was designed to be used with the Magazine Cut Off, a plate that can be pushed across the top of the magazine to prevent the rounds from indexing upwards when the bolt is worked. The idea was that a body of men could engage tightly packed masses of advancing enemy at extreme ranges by firing 'volleys' and loading single shot each time, under the command of their NCO. The 'dropping' of large numbers of heavy 215 grain weight bullets into a mass of men at extreme ranges would start to falter an advance and hopefully cause 'disquiet' among them at least. As the enemy drew closer, the sights would be adjusted, until the enemy were close enough to disengage the Magazine Cut Off and fire more rapidly using the magazine feed over shorter ranges, and using the standard blade and adjustable leaf sight on the top line of the barrel."
Just regarding the .303 rifle you mentioned, I recall seeing this rifle in the Imperial War Museum in London, on display in the Middle-East section