When the Volley Sights originally were devised, the Gatling, Nordenfelt, Gardner and Maxim all were known and actually in Service. The Army had experience with the things, knew their capabilities and they really WANTED them.
Two troubles:
1. the things cost serious money (20 or more times the price of a rifle) and,
2. it had been Parliamentary policy since Cromwell's time to keep the Army as broke as possible, thus avoiding the possibility of England having a military dictatorship for a Government..... again.
Collectors should understand this; it is the reason that there are precious few (if any) Lee-Enfields dated 1936, 1937, 1938, 1939 while there are literally scads of Mausers with these dates: nearly all Lee-Enfield rifles are wartime manufacture.
So the situation was simple: the Army knew what it needed and how to use the things, but did not have the money to buy them. It did, however, have lots and lots of troopies and, at a bob a day per man, they were a lot cheaper to have standing about than a machine-gun which cost several hundred pounds and (very often) required special ammunition.
The SOLUTION lay in the fact that the British Army had excellent fire discipline and the men, in general, were very good on the rifle-range. Rifle practice was encouraged and the standard of accuracy far exceeded that of most other armies of the period. In the end, a solution was devised: add 2 shillings' worth of auxiliary long-range sights to each rifle and, when you need a machine-gun and don't have one, call out a Company or so of troopies, indicate the Target area and call out the orders to blanket that area with rifle-fire, thus creating instantly a beaten zone with plunging fire into which the Enemy, had he any consideration for his own skin, would not venture.
The Volley Sights were stopped in manufacture, then finally removed and later forgotten, in the dark days of Trench warfare when a 300-yard shot became "long range" shooting.
Hope this helps.