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
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.
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.
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 500 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 875 yards 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 M62 is a 7.62x51mm NATO tracer consisting of a 142 gr bullet with 46 grains of WC 846 powder. Orange tip.
The M276 is a dim tracer that uses composition R 440, which is barium peroxide, strontium peroxide, calcium resonate, and magnesium carbonate. Violet tip.