By the start of the First World War, the French military establishment had been experimenting for some time with implementing semiautomatic rifles into line regiments. By 1913 they had decided on the Meunier A6, a long recoil operated rifle to replace the Lebel then in service, however extensive delays occurred due to indecision on ammunition selection, with the eventual choice being for a rimless 7.56x95 round.
Then, the war began, and France had to go to war with the rifles and ammunition already in service.
Two years later, the French adopted what would become the M1917 RSC, a semiautomatic service rifle more cheaply and easily built than the Meunier due to its being built largely of existing Lebel components. It was gas-operated and loaded standard 8mm Lebel rounds in a five-round en bloc clip. French soldiers found it too heavy, too long, and too high-maintenance.
A replacement, the M1918 RSC, was adopted for service in 1918, with the intent that it should eventually be used to replace all other rifles then in service.
In other words, the Ministère des Armées was planning to go fully semiautomatic before 1920.
Then, the war ended, and production of the M1918 RSC was halted with just a few thousand finished. Some of these saw service in the Rif War of the 1920s, where French soldiers issued the rifles gave them good reviews, but they never took their originally intended place as the main service rifle of the Armée de terre.
My scenario is this. Suppose the decision on ammunition for the Meunier did not take so long, and so when the war eventually kicked off it was already in full production. Early in the war many French units would have still had their Lebels, but with the Ministère shipping Meuniers to the lines as quickly as possible. It was a generally well-received rifle, and the ammunition was seen as excellent.
What impact does this have on WWI, and later on WWII?
Alternatively, you could postulate the impact on WWII of the M1918 RSC being pushed into general service between the wars.