The first feasible production firearm designed around a form of smokeless powder was the model 1886 Lebel rifle, which used the Poudre B powder invented by Paul Vieille. It was designed in 1885, and production began in 1887. It fired a 8x50R cartridge.
Several. The 7.7x56R/.303 British, the 7.62x54R, the .30-40 Krag, the 6.5x55 Swedish Service, the 7x57, the 7.92x57, the 8x50R Lebel, etc...
The only rifle it could be is the French Berthier Mannlicher in caliber 8x50R. First developed in 1890 and continued in service until after WWII. It was redesigned before WWI to be a five shot repeater and a lot of original 3 shot models were modified also.
The most common rifle in WWI would have been: * the Russion 7.62R Mosin Nagant Model 91; * the British and Commonwealth .303 Lee Enfield No.1 MK III; * the German 7.92 mm Gew98 (later shortened and designated the 98K) Mauser; * the French 8x50R Berthier and Lebel rifles (both used in huge quantities); * the Italian 6.5mm Carcano; * the Austrian 8x50R Steyr-Mannlicher M1895; and, * the American 30.06 Springfield (M1903) and P17 Enfield. Additonally, both some South African troops in central Africa and Portugese troops in both Africa and Europe used the 7 mm Mauser, typically in the Gew88 pattern. Regards Richard Wm ShortOttawa - Canada
Most likely, the Hotchkiss Mle 1914, which was French in origin and utilised the French 8x50R Lebel cartridge. Although the Browning M1917 was supposed to be the standard machine gun of the US Army, the Army never showed much interest in it until the outbreak of the war, and were forced to purchase foreign weapons as the M1917 could not be manufactured fast enough to meet the sudden demand.