[10] The onset of the First World War increased demand for the Lewis gun, and BSA began production, under the designation "Model 1914".

[12] all manufacture was carried out by BSA in England and the Savage Arms Company in the US,[13] and although the two versions were largely similar, enough differences existed to stop them being completely interchangeable, although this had been rectified by the time of the Second World War.

The piston was fitted with a vertical post at its rear which rode in a helical cam track in the bolt, rotating it at the end of its travel nearest the breech.

Some discussion occurred over whether the shroud was necessary: in the Second World War, many old aircraft guns that did not have the tubing were issued to anti-aircraft units of the British Home Guard and to British airfields, and others were used on vehicle mounts in the Western Desert; all were found to function properly without it, which led to the suggestion that Lewis had insisted on the cooling arrangement largely to show that his design was different from Maclean's earlier prototypes.

A recoil enhancer was added to the 1918 aircraft gun variant (and refitted to many 1917 models) which increased the rate of fire to about 800 rounds per minute.

Light Infantry Pattern Lewis Gun", which lacked the aluminium barrel shroud and had a wooden fore grip) designed as a form of automatic rifle.

Notes made during his training in 1918 by Arthur Bullock, a private soldier in the 2/4th Oxfordshire and Buckinghamshire Light Infantry, record that the chief advantage of the gun was 'its invulnerability' and its chief disadvantages were 'its delicacy, the fact that it is useless for setting up a barrage, and also that the system of air cooling employed does not allow of more than 12 magazines being fired continuously'.

The Lewis guns supplied by Britain were dispatched to Russia in May 1917, but it is not known for certain whether these were the Savage-made weapons being trans-shipped through the UK, or a separate batch of UK-produced units.

[42] They used captured Lewis guns in both World Wars, and included instruction in its operation and care as part of their machine-gun crew training.

The Lewis's popularity as an aircraft machine gun was partly due to its low weight, the fact that it was air-cooled and that it used self-contained 97-round drum magazines.

[45] It was also fitted on two early production examples of the Bristol Scout C aircraft by Lanoe Hawker in mid-1915, mounted on the port side and firing forwards and outwards at a 30° angle to avoid the propeller arc.

The problem in mounting a Lewis to fire forward in most single-engined tractor configuration fighters was due to the open bolt firing cycle of the Lewis, which prevented it from being synchronized to fire directly forward through the propeller arc of such aircraft; only the unusual French SPAD S.A "pulpit plane" which possessed a unique hinged gunner's nacelle immediately ahead of the propeller (and the pilot), and the British pusher fighters Vickers F.B.5, Airco D.H.2, Royal Aircraft Factory F.E.2 and F.E.8 could readily use the Lewis as direct forward-firing armament early in the war.

[46] Until September 1916 Zeppelin airships were very difficult to attack successfully at high altitude, although this also made accurate bombing impossible.

Aeroplanes struggled to reach a typical altitude of 10,000 feet (3,000 m), and firing the solid bullets usually used by aircraft Lewis guns was ineffectual: they made small holes causing inconsequential gas leaks.

Britain developed new bullets, the Brock containing spontaneously igniting potassium chlorate,[47] and the Buckingham filled with pyrophoric phosphorus,[48] to set fire to the Zeppelin's hydrogen.

The Foster mount usually incorporated an arc-shaped I-beam rail as its rearmost structural member, that a Lewis gun could be slid backwards and downwards along the rail towards the cockpit, to allow the ammunition drum to be changed in flight – but RFC fighter ace Albert Ball VC also understood that the Lewis gun in such a mount also retained its original trigger, and could thus be fired upwards.

In the crisis following the Fall of France, where a large part of the British Army's equipment had been lost up to and at Dunkirk, stocks of Lewis guns in both .303 and .30-06 were hurriedly pressed back into service, primarily for Home Guard, airfield defence and anti-aircraft use.

[54] In addition to their reserve weapon role in the UK, they also saw front-line use with the Dutch, British, Australian, and New Zealand forces in the early years of the Pacific campaign against the Japanese.

[58] At the start of the Second World War, the Lewis was the Royal Navy's standard close-range air defence weapon.

[61][62] The Lewis was officially withdrawn from British service in 1946,[36] but continued to be used by forces operating against the United Nations in the Korean War.

[37] A commercial venture in 1921 by the Birmingham Small Arms Company was a version which fired the 12.7×81mm (0.5-inch Vickers) ammunition, intended for use against aircraft and tanks.

At around the same time, BSA developed the Light Infantry Model which had a 22-round magazine and a wooden fore-stock in place of the radiator fins and shroud; it was intended to be used in a similar way to the Browning Automatic Rifle.

Another development was a twin Lewis for aircraft use in which the bodies of the two weapons were joined side-by-side and the drum magazines were mounted vertically, one on each side.

[71] Lewis had also experimented with lighter, 30-06 calibre, box magazine-fed infantry rifle variants intended for shoulder or hip fire as a competition to the BAR.

Despite being three pounds lighter than it and loaded with very forward-thinking features for the time (such as an ambidexterous magazine release), the U.S. Army still chose to adopt the BAR.