The Chatellerault FM Mle 1924-29 was the French military’s replacement for the 1915 Chauchat automatic rifle. It was clear after WWI that the Chauchat design was flawed, and a new weapon was required. The military high command had also made the decision to change to a rimless cartridge. After testing of all the major designs available (Madsen, BAR, Hotchkiss, Lewis and Berthier), the Chatellerault was designed with the best parts of the Hotchkiss and BAR. It fed from a 25-round magazine, with a deliberately slow 450 RPM rate of fire.
The gun was adopted in 1924 using a 7.5x58mm cartridge. It was discovered, however, that the German 8mm Mauser cartridge would chamber and fire in the gun – destroying it in the process. This was a problem, as significant numbers of captured German machine guns in 8mm were in use by the French military at the time and the two cartridges looked fairly similar. The solution arrived at was to change the French standard round to 7.5x54mm, resulting in a chamber too short to accept any other military cartridge. This modification was designated the Mle 1924-29, and by the end of 1934 all existing guns had been converted to the new cartridge.
Chatellerault FM Mle 1924 (download whole gallery in high res)