Tracer ammunition

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. Tracer fire can also be used to signal to other shooters where to concentrate their fire during battle. 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. Tracer fire can also be used to signal to other shooters where to concentrate their fire during battle. 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 shooters that their weapons are 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. However, this practice similarly alerted astute enemies that their foes were nearly 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 their 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 its 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 in 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. Before the development of tracers, gunners relied on seeing their bullet impacts to adjust their aim. However, these were not always visible, especially as the effective range of ammunition increased dramatically during the later half of the 19th century, meaning the bullets could impact a mile or more away in long range area fire. 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 hit 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 reconnaissance, 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 US naval and marine aircrew were issued tracer rounds with their side arms for emergency signaling use as well as defense. A tracer projectile is constructed with a hollow base filled with a pyrotechnic flare material, made of a mixture of a very finely ground metallic fuel, oxidizer, and a small amount of organic fuel. Metallic fuels include magnesium, aluminum, and occasionally zirconium. The oxidizer is a salt molecule which contains oxygen combined with a specific atom responsible for the desired color output. Upon ignition, the heated salt releases its oxygen to sustain combustion of the fuel in the mixture. The color-emitting atom in the salt is also released and reacts chemically with excess oxygen providing the source of the colored flame. In NATO standard ammunition (including US), the oxidizer salt is usually a mixture of strontium compounds (nitrate, peroxide, etc.) and the metallic fuel is magnesium. Burning strontium yields a bright red light. Russian and Chinese tracer ammunition generates green light using barium salts. An oxidizer and metallic fuel alone, however, do not make a practical pyrotechnic for the purposes of producing colored light. The reaction is too energetic, consuming all materials in one big flash of white light – white light being the characteristic output of magnesium-oxide (MgO), for example. Therefore, in the case of using strontium nitrate and magnesium, to produce a red colored flame that is not over-powered by the white light from the burning fuel, a chlorine donor is provided in the pyrotechnic mixture, so that strontium chloride can also form in the flame, cooling it so that the white light of MgO is greatly reduced. Cooling the flame in this manner also lengthens the reaction rate so that the mixture has an appreciable burn time. Polyvinyl chloride (PVC) is a typical organic fuel in colored light for this purpose. Some modern designs use compositions that produce little to no visible light and radiate mainly in infrared, being visible only on night vision equipment. There are three types of tracers: bright tracer, subdued tracer and dim tracer. Bright tracers are the standard type, which start burning very shortly 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.