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The "Modifié 1916" update to the Berthier system of rifles and carbines marked a major improvement in the guns combat effectiveness - really the first substantial overhaul to the design since it was developed in 1890. The two main elements of the upgrade were the addition of an upper handguard and an extension of the magazine from 3 rounds to 5, to match the capacity of the Mauser rifles used by Germany. In addition, several other improvements were made at the same time, including a redesign of the sights to favor quick target acquisition over long range precision and the addition of luminous radium elements to the sights. The upgrade package originally also included a dust cover over the bolt, but this was dropped for reasons I have not been able to determine.
This upgrade package was formally adopted in late 1916, and would go into production in 1917. However, it took a substantial time for the weapons to filter down to the front lines, and only a small number of M16 carbines and a very small number of M16 long rifles actually saw combat service before the Armistice. The M16 pattern (particularly the carbines) would form the standard armament for the French military right up to World War II, however, with Berthier carbine production continuing until 1939.