Tracer ammunition (tracers) are special bullets that are modified to accept a small pyrotechnic charge in their base. Ignited upon firing, the composition burns very brightly, making the projectile visible to the naked eye. This enables the shooter to follow the bullet trajectory relative to the target in order to make corrections to his or her aim.
When used, US 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 his or her weapon is almost empty.
Tracerfire on Finnish-Soviet border during the Winter War 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 Convention's 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 British introduced a tracer version of the .303 cartridge in 1915.The USA introduced a .30-06 tracer in 1917. Prior to adopting red and, later, other color bullet tips, for tracers, US 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.
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 US and NATO standard ammunition, this is usually a mixture of strontium compounds (nitrate, peroxide,...) and a metal fuel such as magnesium. This yields a bright red light. Russian and Chinese tracer ammunition generates red or green light using barium salts. It is not true that they use only green tracers even if identified by green bullet tips. 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 even weight from ordinary rounds. Over long ranges, the stream of tracer rounds and the stream of ordinary rounds will diverge significantly, due to a tracer bullet's mass decreasing over time, because the tracer material in its base burns and vaporizes. Although advances in tracer design have diminished this problem, it still exists in modern ammunition.
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.
A recent patent U.S. Patent Application 20040099173 covers the use of an LED and capacitor, instead of a pyrotechnic compound, in an attempt to stop the tracer being seen from the front. As an additional benefit, such tracer rounds would keep a constant mass during their flight and thus, keep to a more predictable trajectory. Furthermore, an LED and capacitor would probably be able to emit light considerably longer than conventional tracer bullets can; 7.62 x 51mm, 5.56x45, and 7.62 x 54mm tracers burn out at 900 meters, 5.45 x 39mm tracers burn out at 300 meters or less.
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 900 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 900 meters and has a red tip (orange when linked 4 to 1 with the M249).
The M25 is a 30.06 tracer cartridge consisting of a 145 gr bullet with 50 grains of IMR 4895 powder. Orange tip. The tracer compound contains composition R 321 which is 16% polyvinyl chloride, 26% magnesium powder, 52% strontium nitrate.
The M62 is a 7.62x51mm NATO tracer consisting of a 142 gr bullet with 46 grains of WC 846 powder. Orange tip. 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.)
The M276 is a dim tracer that uses composition R 440, which is barium peroxide, strontium peroxide, calcium resinate for example calcium abietate, and magnesium carbonate. Violet tip.
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.
Another application for tracers is in tanks, where they are fired from a co-axial machine gun to indicate where a round from the main gun will impact. This practice also saw use in World War II in air combat, where some aircraft were equipped with both machine guns and autocannons. Tracer fire from the machine guns would be used to align correctly the attacking aircraft, so that shots from their cannon would be most effective.
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 less essential.
In the UK, usage of tracer rounds is restricted on NRA-operated ranges, due to an increased risk of fire. Unauthorized use will be 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 tracing 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.