Flare Guns were single-shot brass guns carried by assault teams, sentries, and pilots. The combustion of different chemical ingredients with a fuel source like magnesium would be fired into the air, where the small cartridge would burn out in a set color.
During an assault, soldiers would shoot up a flare to signal their commanders that they had successfully reached an objective or infiltrated a trench. Flares were used by pilots to point out strong points and valuable targets for their artillery. They were also used by sentries when spotting movement during the night. Once brightly illuminated, men sneaking through the battlefield suddenly became big targets.