Tracer bullets have a hollow back, filled with a pyrotechnic flare material. Usually this is a mixture of magnesium, perchlorate, and chromium, to yield a bright red color. Tracers allow the shooter to get a good idea where most of the bullets are going, without actually having to look through a sighting apparatus.
Tracers were used extensively in machine guns in World War I, and usually loaded every 4 rounds in ground guns, and every 2 or 3 rounds in aircraft guns.
Tracers were used extensively in World War I and the Vietnam war (by the Vietnamese). Tracers are used by the U.S. marines in nightfire situations.
A disadvantage of tracers is that they show the enemy the location of the shooter. As an old military proverb puts it, tracers work both ways.
Early tracer rounds often had different aerodynamics and even weight from ordinary rounds, so that over long ranges the stream of tracer rounds and the stream of ordinary rounds could diverge. This is less of a problem with modern tracer rounds.
