Tracer ammunition

Tracer ammunition (tracers) use 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 aim.

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 shooter'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' that 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 correctly align the attacking aircraft so that fires 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.

Tracers are usually loaded between one in four rounds to one in six rounds. Platoon leaders will sometimes load their magazines entirely with tracers to mark targets for their men to fire on.

For those on the receiving end of tracer ammunition, there is a well-known optical illusion whereby the tracer rounds appear to be travelling slowly, but as they get closer they speed up considerably.

History
Before the development of tracers, gunners would rely 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 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 US introduced a 30-06 tracer in 1917.

Construction
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 salts and a metal fuel such as magnesium. This yields a bright red light. Russian and Chinese tracer ammunition generates green light using barium salts.

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 radically, especially given that a tracer bullet's mass decreases 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.

Types
There are three types of tracers: bright tracer, subdued tracer and dim tracer. The standard tracer starts 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 less useful. 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 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. However, this benefit may be offset by the fact that such bullets would probably have a very different weight than normal bullets. Furthermore, an LED and capacitor would probably be able to emit light considerably longer than conventional tracer bullets can; 7.62 x 51 mm or 7.62 x 54 mm tracers burn out at 800 meters and 5.56 x 45 mm or 5.45 x 39 mm tracers burn out at 300 meters or less.

The M856 tracer cartridge (63.7-grain bullet) is used in the M16A2/3/4, M4-series, M249 weapons (among other 5.56-mm 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 isn't sufficient to stabilize the projectile.

The M196 tracer cartridge (55-grain bullet) is another tracer round for 5.56 NATO weapons, but it is just used for training purposes. It has a red tip and is designed to trace out to 500 yards.

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).