Tracer ammunition
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Tracer ammunition (tracers) use special bullets that are either white phosphorus-tipped or contain a powder in their base, possibly magnesium phospate, that burns very brightly during their flight making them visible to the naked eye. This enables the shooter to follow the bullet trajectory relative to the target in order to make corrections to their aim.
Tracers can also serve to direct fire at a given target, because it is visible to other combatants. The disadvantages is that they also betray the shooter's position. Its lethality is similar to conventional ammunition. The mass loss and the burning aspects can make the consequences of the impact slightly different though.
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). The Soviet Air Force during World War II also used this practice for aircraft machine guns. It has been said that a disadvantage in this practice is that the enemy is alerted that the pilot or shooter is low on ammunition and possibly vulnerable. However, it is generally agreed upon that for ground forces, this offers no tactical advantage to the enemy since a solider is supposed to alert his team that he is "dry", and rely on their support while he reloads. Thus, an enemy must expose himself in order to attack the reloading soldier. In the air, fights rarely involve firearms. Instead, modern aircraft tend to rely on missiles.
Tracers are usually limited to between one in four rounds to one in six rounds. This is to prevent the buildup of incendiary byproducts in the barrel of the weapon over time, hindering its performance and eventually damaging it. 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.
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[edit] History
Tracers have been used extensively in machine guns since World War I (1914-1918) and are usually loaded at a ratio of one tracer per four rounds in ground guns, and one tracer per every two or three rounds in aircraft guns, although with most emplaced machine guns, such as the M60, it is common to have a tracer every 6 rounds.
[edit] Construction
A tracer projectile is constructed with a hollow base filled with a pyrotechnic flare material. In US and NATO standard ammunition this is usually a mixture of strontium salts and a metal fuel such as magnesium perchlorate. 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.
[edit] 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 an old military proverb 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 (US 2004/99173) 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.
A possible solution would be to put a Fresnel lens over the back of the bullet, as seen on many traffic lights, to reduce the observable angle, but that would only work if the tracer material were burning in the rear of the bullet alone instead of in a long narrow cloud extending several feet behind the bullet as it travels.
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 due to the fact that 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 a bit longer than the M196).
