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When World War One broke out in 1914, France mobilized millions of men into military service - and it became abundantly clear that a lot of new rifles would need to be manufactured. The 1886 Lebel was no longer in production and was a slow rifle to make in any case - but the 1907 Colonial Berthier was being produced already, and was a more efficient design as well. By late 1914, plans were underway to drastically increase Berthier rifle production.
A few changes were made to the 1907 pattern, mainly changing the front end to accept the standard 1886 Lebel bayonet ("Rosalie"). After the first 80,000 had been made, the bolt handle was also changed from the bent carbine style to a heavier duty straight type. With these changes made, the Modele 07/15 was ready for mass adoption by the infantry, where it would serve side by side with the Lebel.
The primary manufacturer of 07/15 rifles was the St. Etienne arsenal, which build between 1.0 and 1.2 million of them by the spring of 1917. The Chatellerault arsenal produced another 436,000, and the Delauney Belleville automobile factory retooled its workshops to make rifles, producing another 170,000 of them. The Remington company in the US also took a contract to make 250,000 Berthier rifles, but was unable to meet the terms of the contract. Remington had taken on more wartime production work than it could handle, and failed to meet French quality and scheduling requirements. In August of 1916 that contract was cancelled, with just 9,440 rifles sent to France for use. The remaining 5,000-10,000 that had been made by Remington were sold on the commercial market in the US.
By 1916, it was clear that the 3-round capacity of the Berthier was a very real disadvantage (both tactically and psychologically) to the German Mauser rifles, and an upgrade program was put underway. This would ultimately become the 1916 pattern and would go into production in 1917.