It seems that armies are always short of weaponry when major conflicts light up, and creative solutions are often the result. Canada during the First World War is a good example. They had a serious shortage of light machine guns, and a Quebecois engineer named Joseph Huot devised a way to modify existing Ross straight-pull rifles into drum-fed light machine guns.
He added a gas piston to operate the bolt (similar to the later Enfield conversions like the Howell, but simpler because of the straight-pull nature of the Ross), changed the box magazine to a 25-round drum, added a cooling shroud like the Lewis, and a guard at the back to protect the shooter from the cycling bolt.
The Huot automatic rifle was tested against other available light machine guns like the Lewis and Farquhar-Hill, and proved surprisingly successful. It was very reliable, even in poor conditions, and was popular with soldiers who used it. Best of all from the government’s view, it was far cheaper to manufacture than the Lewis. An order for several thousand was requested, but the war ended before it could be filled, and the idea was dropped.
If I can get my hands of a set of Huot drawings or an original rifle to disassemble and photograph, I think this would be a terrific project to build, starting with a relatively cheap sporterized Ross rifle. Not something with much mass appeal, but it sure would be neat to be able to shoot one.