Incendiary ammunition

Incendiary ammunition is a type of firearm ammunition containing a compound that burns rapidly and causes fires.

World War I
The first time incendiary ammunition was widely used was in World War I. At the time, phosphorus was the primary ingredient in the incendiary charge and ignited upon firing, leaving a trail of blue smoke. These early forms were also known as "smoke tracers" because of this. Though deadly, the effective range of these bullets was only 350 yards (320 m), as the phosphorus charge burned quickly. Incendiary rounds called "Buckingham" ammunition were supplied to early British night fighters for use against military zeppelins threatening the British Isles. The flammable hydrogen gas of the zeppelins was susceptible to fire, making incendiary rounds much more deadly than standard ones which would pass through the outer skin without igniting the gas. Similarly, incendiary ammunition was used against non-rigid observation balloons. The British Royal Flying Corps forbade the use of incendiary rounds for air-to-air combat with another airplane, as their use against personnel was at first considered to be a violation of the St. Petersburg Declaration. Pilots were only permitted to deploy them against zeppelins and balloons, and they were additionally restricted to shooting the gas chamber itself rather than the crew. Furthermore, they were required to carry written orders on their person when engaging these targets.

World War II
During World War II, incendiary rounds found a new use: they became one of the preferred types of ammunition for use in interceptor fighters. They were not nearly as effective at puncturing enemy bomber aircraft as armor piercing rounds, but were far more effective than standard rounds because they could also ignite fuel if they came into contact with a fuel tank or pipeline. Incendiary rounds were developed in Britain following the failure of a Swiss-developed incendiary (De Wilde ammunition) when it was tested for use in the Hawker Hurricane and Supermarine Spitfire. A round with similar incendiary capabilities was developed by Major Dixon at the Woolwich Arsenal, and was adopted by British forces as the 0.303 Incendiary B Mark VI. It was initially called De Wilde ammunition, even though the design was almost entirely different from the original Swiss version. The B Mark VI incendiary bullet was packed with nitrocellulose, and a small steel ball was placed in the tip of the bullet to ensure that the chemical explode on impact. As opposed to earlier designs, the M Mark VI was a true incendiary round rather than a tracer round.

One fighter pilot who was shot down by incendiary rounds while flying in the Battle of Britain describes his experience:

Modern
Incendiary projectiles, in particular those intended for armor penetration, are more effective if they explode after penetrating a surface layer, such that they explode inside the target. Additionally, targets with onboard electronics or computers can be damaged by metal fragments when they explode on the surface. Ignition is often delayed by varying means until after impact.

Some explosive projectiles, such as high-explosive incendiary rounds, contain an incendiary charge intended to ignite explosives within the shell.