Before I go any further, if you are seriously interested in the history and workings of the Lewis gun, check out C&Rsenal’s video. If you just want a quick overview, you can skip that and carry on reading…
Issac Newton Lewis, Colonel, US Army designed his machine gun in 1911 and spent around two years banging his head against a brick wall trying to persuade the US Ordinance Department to adopt his gun. In 1913 he left the United States and set up shop in Belgium where he received a respectable first order from the Belgians. In 1914 BSA bought a license to produce it and as war loomed Lewis moved his factory to Britain to keep it out of German hands. Lewis’ BSA license proved very profitable: between BSA and Savage Arms around 50,000 Lewis Guns were produced by 1918 and the license granted him commission on every one made.
The design itself was based on work done by Samuel Maclean, but between Lewis and designers at BSA it was transformed into a reliable and easy to produce machine. It is gas operated, open bolt with three locking lugs at the rear of the bolt. The most distinctive feature is of course the massive aluminium heat sink/fins/barrel jacket arrangement.
Shot of the Lewis receiver. On the right you can see the barrel jacket and the rear end of the cooling fins. On the bottom of the gun, forward to the trigger is the clock-type main spring.
The idea of this was to wick heat away from the barrel as quickly as possible (quick-change barrels weren’t really a thing yet). The large mass of aluminium took the heat into the fins and the muzzle blast would suck air through from the back towards the front. In theory. In reality the necessity of this sophisticated arrangement is dubious, aircraft Lewis Guns were pressed into service on the ground during WWII, even in North Africa and the guns were found to function perfectly well without.
The magazine is also worth mentioning, coming in 47 and 97 round versions. This Pan magazine is NOT a drum magazine (which relies on a spring to feed, keeping ammunition aligned with the bore) and is manually rotated and indexed as the gun operates.
The indexing system for the Lewis. If you want to know how it works, see the C&R video above for the animation.
Last but not least, the mainspring is a spiral clockwork type, mounted in that distinct protrusion from the belly of the receiver. This is very efficient and saves space over a much larger conventional mainspring, also allowing for easy adjustment to account for field conditions to make for reliable firing in all weather.
The Lewis gun’s service was long and varied. Before even leaving for Europe, Lewis had put the gun on a Wright Flyer and as a result it has the distinction of being the first machine-gun fired from an aircraft (1912).
During the Great War, Britain used them extensively, eventually outnumbering the Vickers by about 3:1 in spite of being more expensive. That the government was willing to spend so much more on these than an established home-grown piece is a comment on the quality of the design.
Due to its light weight, the Lewis was readily adopted by Air Arms where it was widely used for observer’s defensive guns. It was also mounted as foreward firing guns but had to be mounted outside of the propeller’s arc due to firing from an open bolt (therefore being nigh-on impossible to synchronise to a propeller).
By the Second World War, the Lewis was outdated for the role in which it was originally used. After Dunkirk and the fall of France, Britain pulled its Lewis guns out of reserve where it was used by the Home Guard and for low-level anti-aircraft fire. In the Far East it served with Empire forces on the front lines. Throughout the war it remained in use with the Navy and RAF for air defence from boats.
As well as versions chambered in .303 British, there were Lewis guns made in 7.92 and 7.7 Japanese rimmed, the Japanese having copied the design from versions captured in the Far East.
The Lewis had some limited influence on other designs, the FG42 taking inspiration for its bolt/piston arrangement and from that the M60. There was also a Lewis pistol, which fired from an open bolt.
Lewis gun firing in slow motion
Lewis Gun video, manuals and pictures
The Lewis Pistol