Tracer ammunition
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Tracers are special bullets that are white phosphorus-tipped or contain a powder in their base that burns very brightly during their flight. This enables the shooter to follow the bullets' trajectories. The shooter then, typically, "walks" his cone of fire onto the target by seeing where the tracer is going. 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.
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.
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 needs changing, particularly when using a weapon (such as an AK-47) that does not lock the bolt back when empty. The Soviet Air Force during World War II also used this practice for aircraft machine guns. Of course this often alerted the enemy that the pilot in question was low on ammo and thus vulnerable.
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 tracer can also overwhelm night vision devices, rendering them less useful. Subdued tracer burns at full brightness after a hundred or more yards to avoid giving away the gunner's position. Dim tracer burns very dimly but is 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; 7.62x51mm or 7.62x54mm tracers burn out at 800 meters and 5.56x45mm or 5.45x39mm tracers burn out at 300 meters or less.
A possible solution would be to put a grill over the back of the bullet, as seen on many traffic lights, to cut 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 burning rapidly for several feet behind the bullet as it travels.
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.
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). The M856 is designed to trace out to 875 yards. The M856 has a red tip (orange when linked to 4 to 1 for 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 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).
References
http://www.btammolabs.com/tests/7.htm
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