When a bullet exits a rifle barrel, it can be traveling between 800 and 1,000 meters per second (2,625 to 3,280 feet per second) -- much too fast to be seen with the naked eye. In the days of black powder, a fired bullet sometimes left a trail of smoke, marking the path of the projectile through the air. But with the advent of smokeless powder, shooters received no feedback about a bullet's trajectory until it arrived at the target.
Enter the tracer round, which includes an additional incendiary compound, usually a phosphorus or magnesium mixture, in the base of the bullet. When a tracer is fired, the powder in the cartridge both propels the bullet and lights the incendiary mixture. As the bullet travels through the air, it gives off an intense light and trails smoke, helping the shooter see the bullet go downrange. Military forces often use this type of ammunition in machine guns, in which every fifth round in the magazine or belt includes a tracer.
Today, tracers can produce a variety of colors for daytime and nighttime applications. White tracers can be seen during the day, while red and green ones can be seen at night.