Benedict Crowell,
America's Munitions 1917-1918
“ The tracer bullet which we manufactured contained a mixture of barium peroxide and magnesium and in flight burned with the intensity of a calcium light. These bullets were principally used by machine gunners of aircraft, since in the air it is impossible to tell where machine-gun projectiles are going unless there is some device enabling the gunner to see the trajectory of the bullets. This is done by inserting tracer bullets at intervals in the belts of cartridges fed into the machine gun. The common conception of a tracer bullet is one that leaves a trace of smoke in its flight ”
