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 visible to the naked eye. This enables the shooter to follow the projectile trajectory to make aiming corrections.
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 sometimes load their magazines entirely with 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.
Tracer rounds may also ignite flammable substances on contact from a nominal distance.
Before the development of tracers, gunners relied on seeing their bullet impacts to adjust their aim. However, these were not always visible. In the early 20th century, ammunition designers developed "spotlight" bullets, which would create a flash or smoke puff on impact to increase their visibility. However, these projectiles were deemed in violation of the Hague Conventions' prohibition of "exploding bullets." This strategy was also useless when firing at aircraft, as there was nothing for the projectiles to impact on if they missed the target. Designers also developed bullets that would trail white smoke. However, these designs required an excessive amount of mass loss to generate a satisfactory trail. The loss of mass en route to the target severely affected the bullet's ballistics.
The United Kingdom was the first to develop and introduce a tracer round, a version of the .303 cartridge in 1915. The United States introduced a .30-06 tracer in 1917. Prior to adopting red and, later, other color bullet tips, for tracers, American tracers were identified by blackened cartridge cases.
Tracers proved useful as a countermeasure against Zeppelins used by Germany during World War I. The airships were used for surveillance and bombing operations. Normal bullets merely had the effect of causing a slow leak, but tracers could ignite the hydrogen gasbags, and bring down the airship quickly.
In World War II U.S. naval and marine aircrew were issued tracer rounds with their sidearms for emergency signaling use as well as defense.
A tracer projectile is constructed with a hollow base filled with a pyrotechnic flare material, often made of phosphorus or magnesium or other bright burning chemicals. In NATO standard ammunition (including U.S.), this is usually a mixture of strontium compounds (nitrate, peroxide, etc.) and a metal fuel such as magnesium. This yields a bright red light. Russian and Chinese tracer ammunition generates green light using barium salts. Some modern designs use compositions that produce little to no visible light and radiate mainly in infrared, being visible only on night vision equipment.
Tracers can never be a totally reliable indicator of a gunner's aim, since all tracer rounds have different aerodynamics and weight from ordinary rounds. Over long ranges, the stream of tracer rounds and the stream of ordinary rounds will diverge significantly. This is caused by a tracer bullet's mass decreasing during flight, because the tracer material in its base burns and vaporizes. Although advances in tracer design have diminished this problem, it cannot be completely eliminated.
There are three types of tracers: bright tracer, subdued tracer and dim tracer. Bright tracers are the standard type, which start burning immediately after exiting the muzzle. A disadvantage of bright tracers is that they give away the shooter's location to the enemy; as a military adage puts it, "tracers work both ways". Bright tracers can also overwhelm night-vision devices, rendering them useless. Subdued tracers burn at full brightness after a hundred or more yards to avoid giving away the gunner's position. Dim tracers burn very dimly but are clearly visible through night-vision equipment.
The M196 tracer cartridge (55-grain bullet) is a training round for 5.56mm NATO weapons. It has a red tip and is designed to trace out to 500 yards.
The M856 tracer cartridge (63.7-grain bullet) is used in the M16A2/3/4, M4-series, and M249 weapons (among other 5.56mm NATO weapons). This round is designed to trace out to 875 yards and has a red tip (orange when linked 4 to 1 with the M249). It is not to be used in the M16A1 except under emergency conditions and at ranges of less than 90 meters, because the M16A1's rifling twist is not sufficient to stabilize the projectile. The M16A2 rifle has a rifling twist of 1 in 7" to stabilize the M856 tracer rounds (since the M856 is slightly longer than the M196).
The M25 is an orange-tipped 30.06 tracer cartridge consisting of a 145 gr bullet with 50 grains of IMR 4895 powder. The tracer compound contains composition R 321 which is 16% polyvinyl chloride, 26% magnesium powder, 52% strontium nitrate.
The M62 is an orange-tipped 7.62x51mm NATO tracer consisting of a 142 gr bullet with 46 grains of WC 846 powder. The tracer compound contains composition R 284 which is 17% polyvinyl chloride, 28% magnesium powder, and 55% strontium nitrate. (This is the same composition used on the M196.)
Tracer compositions can also emit primarily in infrared, for use with night-vision devices. An example composition is boron, potassium perchlorate, sodium salicylate, iron carbonate or magnesium carbonate (as combustion retardant), and binder. Many variants exist.
Tracers can also serve to direct fire at a given target, because it is visible to other combatants. The disadvantage is that they betray the gunner's position; the tracer path leads back to its source. To make it more difficult for an enemy to do this, most modern tracers have a delay element, which results in the trace becoming visible some distance from the muzzle. Its lethality is similar to conventional ammunition. However, the mass loss and the burning aspects can make the consequences of the impact slightly different.
Besides guiding the shooter's direction of fire, tracer rounds can also be loaded at the end of a magazine to remind the shooter that the magazine is almost empty. This is particularly useful in weapons that do not lock the bolt back when empty (such as the AK-47). During World War II, the Soviet Air Force also used this practice for aircraft machine guns. One disadvantage in this practice is that the enemy is alerted that the pilot or shooter is low on ammunition and possibly vulnerable. For ground forces, this generally offers no tactical advantage to the enemy, since a soldier who is out of ammunition is supposed to alert his team that he is "dry" and rely on their support while he reloads. Thus, an enemy must risk exposing himself in order to attack the reloading soldier. Modern aircraft tend to rely on missiles, radar, or laser-guided sights to track the enemy, thereby making the use of tracers inessential.
In the UK, usage of tracer rounds is restricted on National Rifle Association of the United Kingdom-operated ranges, due to an increased risk of fire. Unauthorized use is punished at the discretion of the acting range officer. Use of tracers is usually only authorized during military training.
In July 2009, a large fire was started by tracer ammunition near Marseille, France, an area where shrub vegetation is very dry and flammable in the summer, and where normally this kind of ammunition should not be used.
On February 24th, 2013 a fire was started at DFW Gun Club in Dallas, TX by the use of a tracer round inside the facility. 
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