Apparently making it's first printed appearance in Harry Craddock's "The Savoy Cocktail Book" (1930), this drink is named after a 75-millimeter field gun that French forces and the American national Guard used during the First World War. While with "French" in the name, you might think this drink should really use brandy or cognac as the base spirit, but this drink is properly made with gin.
Shake gin, lemon juice, and sugar with ice.
Strain into ice filled collins glass.
Fill with champagne.