What is "tracer ammunition"

Tracer \Tra"cer\, n. One who, or that which, traces.

2. A person engaged (esp. in the express or railway service) in tracing, or searching out, missing articles, as packages or freight cars.

3. An inquiry sent out (esp. in transportation service) for a missing article, as a letter or an express package.

4. (Mil.) a type of ammunition that emits light or smoke as it moves toward its target, providing a visible path of the projectile in flight so that the point of impact may be observed; -- called also tracer ammunition.

5. (Mil.) the chemical substance used in tracer ammunition to cause it to be visible in flight.

6. a chemical substance with properties, such as radioactivity or fluorescence, which make it easily measurable, used to observe the movements of chemically related substances through a biological, physical, or chemical system; -- in biochemistry, also called labeled compounds.

Note: Radioactive tracers are used, for example, to measure the retention or distribution of residues of drugs after administration to an animal, to determine the type and rate of metabolism; also, to measure the rate of motion of molecules in electrophoresis or the leakage of small quantities of material from a container. Small fluorescent tracers may be attached in many cases to macromolecules such as proteins or nucleic acids, allowing the motions of such macromolecules to be easily observed by their acquired fluorescence, without appreciably changing their properties. In biological and biochemial systems the common radioactive isotopes used in tracers are carbon-14, tritium (hydrogen-3), sulfur-35, phosphorus-32, and iodine-131; other isotopes are also used, including non-radioactive isotopes such as carbon-13.

Tracer ammunition (tracers) are bullets or cannon caliber projectiles that are built with a small pyrotechnic charge in their base. Ignited by the burning powder, the pyrotechnic composition burns very brightly, making the projectile trajectory visible to the naked eye during daylight, and very bright during nighttime firing. This enables the shooter to make aiming corrections without observing the impact of the rounds fired and without using the sights of the weapon.

When used, tracers are usually loaded as every fifth round in machine gun belts, referred to as four-to-one tracer. Platoon and squad leaders will load some tracer rounds in their magazine or even use solely tracers to mark targets for their soldiers to fire on. Tracers are also sometimes placed two or three rounds from the bottom of magazines to alert the shooter that their weapon is almost empty. During World War II, aircraft with fixed machine guns or cannons mounted would sometimes have a series of tracer rounds added near the end of the ammunition belts, to alert the pilot that he was almost out of ammunition. More often, however, the entire magazine was loaded four-to-one, on both fixed offensive and flexible defensive guns, to help mitigate the difficulties of aerial gunnery. Tracers were very common on most WWII aircraft, with the exception of night fighters, which needed to be able to attack and shoot down the enemy before they realized they were under attack, and without betraying its own location to the enemy defensive gunners. The United States relied heavily on tracer ammunition for the defensive Browning M2 .50 caliber machine guns on their heavy bombers such as the B-24 Liberator, but they found that during deflection shooting, gunners who tried to aim using the tracers rather than the sights ended up not "leading" the enemy aircraft enough and missing, because of an optical illusion that made it appear that the tracers were striking the aircraft, when it reality they were passing significantly behind it. This proved true of fixed forward firing guns as well. In some cases, gunners were ordered not to use tracer ammunition at all, but in the end, with greater emphasis during training on using the sights and not trusting the tracers, the problem was solved to a large degree.

Tracer rounds can also have a mild incendiary effect, and can ignite flammable substances on contact, provided the tracer compound has started burning and is still burning on impact.