Armour-piercing ammunition

Armour-piercing ammunition is a type of projectile designed to penetrate armour protection, most often including naval armour, body armour, and vehicle armour.
The first, major application of armour-piercing projectiles was to defeat the thick armour carried on many warships and cause damage to their lightly armoured interiors. From the 1920s onwards, armour-piercing weapons were required for anti-tank warfare. AP rounds smaller than 20 mm are intended for lightly armoured targets such as body armour, bulletproof glass, and lightly armoured vehicles.
As tank armour improved during World War II, anti-vehicle rounds began to use a smaller but dense penetrating body within a larger shell, firing at a very-high muzzle velocity. Modern penetrators are long rods of dense material like tungsten or depleted uranium that further improve the terminal ballistics.

Penetration

In the context of weaponry, penetration is the ability of a weapon or projectile to pierce into or through an obstacle. It depends on the specific properties of the bullet, the obstacle, and the angle of impact. The speed of the projectile is particularly important, even in the case of the same kinetic energy.

History

The late 1850s saw the development of the ironclad warship, which carried wrought iron armour of considerable thickness. This armour was practically immune to both the round cast-iron cannonballs then in use and to the recently developed explosive shell.
The first solution to this problem was effected by Major Sir W. Palliser, who, with the Palliser shot, invented a method of hardening the head of the pointed cast-iron shot. By casting the projectile point downwards and forming the head in an iron mold, the hot metal was suddenly chilled and became intensely hard, while the remainder of the mold, being formed of sand, allowed the metal to cool slowly and the body of the shot to be made tough.
These chilled iron shots proved very effective against wrought iron armour but were not serviceable against compound and steel armour, which was first introduced in the 1880s. A new departure, therefore, had to be made, and forged steel rounds with points hardened by water took the place of the Palliser shot. At first, these forged-steel rounds were made of ordinary carbon steel, but as armour improved in quality, the projectiles followed suit.
During the 1890s and subsequently, cemented steel armour became commonplace, initially only on the thicker armour of warships. To combat this, the projectile was formed of steel—forged or cast—containing both nickel and chromium. Another change was the introduction of a soft metal cap over the point of the shell – so called "Makarov tips" invented by Russian admiral Stepan Makarov. This "cap" increased penetration by cushioning some of the impact shock and preventing the armour-piercing point from being damaged before it struck the armour face, or the body of the shell from shattering. It could also help penetration from an oblique angle by keeping the point from deflecting away from the armour face.

World War I

Shot and shell used before and during World War I were generally cast from special chromium steel that was melted in pots. They were forged into shape afterward and then thoroughly annealed, the core bored at the rear and the exterior turned up in a lathe. The projectiles were finished in a similar manner to others described above. The final, or tempering treatment, which gave the required hardness/toughness profile to the projectile body, was a closely guarded secret.
The rear cavity of these projectiles was capable of receiving a small bursting charge of about 2% of the weight of the complete projectile; when this is used, the projectile is called a shell, not a shot. The high-explosive filling of the shell, whether fuzed or unfuzed, had a tendency to explode on striking armour in excess of its ability to perforate.

World War II

During World War II, projectiles used highly alloyed steels containing nickel-chromium-molybdenum, although in Germany, this had to be changed to a silicon-manganese-chromium-based alloy when those grades became scarce. The latter alloy, although able to be hardened to the same level, was more brittle and had a tendency to shatter on striking highly sloped armour. The shattered shot lowered penetration, or resulted in total penetration failure; for armour-piercing high-explosive projectiles, this could result in premature detonation of the high-explosive filling. Advanced and precise methods of differentially hardening a projectile were developed during this period, especially by the German armament industry. The resulting projectiles change gradually from high hardness at the head to high toughness at the rear and were much less likely to fail on impact.
APHE shells for tank guns, although used by most forces of this period, were not used by the British. The only British APHE projectile for tank use in this period was the Shell AP, Mk1 for the 2 pdr anti-tank gun and this was dropped as it was found that the fuze tended to separate from the body during penetration. Even when the fuze did not separate and the system functioned correctly, damage to the interior was little different from the solid shot, and so did not warrant the additional time and cost of producing a shell version. They had been using APHE since the invention of the 1.5% high-explosive Palliser shell in the 1870s and 1880s, and understood the tradeoffs between reliability, damage, percentage of high explosive, and penetration, and deemed reliability and penetration to be most important for tank use. Naval APHE projectiles of this period, being much larger used a bursting charge of about 1–3% of the weight of the complete projectile, but in anti-tank use, the much smaller and higher velocity shells used only about 0.5% e.g. Panzergranate 39 with only 0.2% high-explosive filling. This was due to much higher armour penetration requirements for the size of shell. Therefore, in most APHE shells put to anti-tank use the aim of the bursting charge was to aid the number of fragments produced by the shell after armour penetration, the energy of the fragments coming from the speed of the shell after being fired from a high velocity anti-tank gun, as opposed to its bursting charge. There were some notable exceptions to this, with naval calibre shells put to use as anti-concrete and anti-armour shells, albeit with a much reduced armour penetrating ability. The filling was detonated by a rear-mounted delay fuze. The explosive used in APHE projectiles needs to be highly insensitive to shock to prevent premature detonation. The US forces normally used the explosive Explosive D, otherwise known as ammonium picrate, for this purpose. Other combatant forces of the period used various explosives, suitably desensitized.

Projectile composition and construction

Cap and Ballistic cap

Name	Schematic	Description
AP – Armour-piercing		No cap
APC – Armour-piercing capped
APBC – Armour-piercing ballistic capped
APCBC – Armour-piercing capped ballistic capped

Cap suffixes are traditionally only applied to AP, SAP, APHE and SAPHE-type projectiles configured with caps, for example "APHEBC", though sometimes the HE-suffix on capped APHE and SAPHE projectiles gets omitted. If fitted with a tracer, a "-T" suffix is added.

Penetrator and filling

Armour-piercing non-solid shells

An armour-piercing projectile must withstand the shock of punching through armour plating. Projectiles designed for this purpose have a greatly strengthened body with a specially hardened and shaped nose. One common addition to later projectiles is the use of a softer ring or cap of metal on the nose known as a penetrating cap, or armour-piercing cap. This lowers the initial shock of impact to prevent the rigid projectile from shattering, as well as aiding the contact between the target armour and the nose of the penetrator to prevent the projectile from bouncing off in glancing shots. Ideally, these caps have a blunt profile, which led to the use of a further thin aerodynamic cap to improve long-range ballistics. Armour-piercing shells may contain a small explosive charge known as a "bursting charge". Some smaller-calibre armour-piercing shells have an inert filling or an incendiary charge in place of the bursting charge.

APHE/SAPHE

Armour-piercing high-explosive shells are armour-piercing shells containing an explosive filling, which were initially termed "shell", distinguishing them from non-explosive "shot". This was largely a matter of British usage, relating to the 1877 invention of the first of the type, the Palliser shell with 1.5% high explosive. By the start of World War II, armour-piercing shells with bursting charges were sometimes distinguished by the suffix "HE"; APHE was common in anti-tank shells of 75 mm calibre and larger, due to the similarity with the much larger naval armour-piercing shells already in common use. As the war progressed, ordnance design evolved so that the bursting charges in APHE became ever smaller to non-existent, especially in smaller calibre shells, e.g. Panzergranate 39 with only 0.2% high-explosive filling.
The primary projectile types for modern anti-tank warfare are discarding-sabot kinetic energy penetrators, such as APFSDS. Full-calibre armour-piercing shells are no longer the primary method of conducting anti-tank warfare. They are still in use in artillery above 50 mm calibre, but the tendency is to use semi-armour-piercing high-explosive shells, which have less anti-armour capability but far greater anti-materiel and anti-personnel effects. These still have ballistic caps, hardened bodies and base fuzes, but tend to have far thinner body material and much higher explosive contents.
Common terms for modern armour-piercing and semi-armour-piercing shells are:

HEI-BF – High-explosive incendiary
SAPHE – Semi-armour-piercing high-explosive
SAPHEI – Semi-armour-piercing high-explosive incendiary
SAPHEI-T – Semi-armour-piercing high-explosive incendiary tracer