Armor-piercing shot and shell

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Armour piercing shell of the APBC
1 Light weight ballistic cap
2 Steel alloy piercing shell
3 Desensitized bursting charge (TNT, Trinitrophenol, RDX...)
4 Fuse (set with delay to explode inside the target)
5 Bourrelet (front) and driving band (rear)

An armour piercing shell is a type of ammunition designed to penetrate armour and detonate. In naval warfare and older anti-tank shells, the shell had to withstand the shock of punching through armour plate. Shells designed for this purpose had a greatly strengthened case with a specially hardened and shaped nose, and a much smaller bursting charge. Some smaller calibre AP shells have an inert filling, or incendiary charge in place of the HE bursting charge. The AP shell is now very rarely seen except in naval usage, and is not commonly used there.

1 History
2 First World War Era
3 Second World War
4 Modern Day
5 References

[edit] History

On the introduction of ironclad warships in the late 19th century it was found that the ordinary cast-iron projectile readily pierced the thin plating, and in order to protect the vital parts of the vessel wrought iron armour of considerable thickness was placed on the sides. It then became necessary to produce a projectile which would pierce this armour. This was effected by Sir W. Palliser, who 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 mould, the hot metal was suddenly chilled and became intensely hard, while the remainder of the mould being formed of sand allowed the metal to cool slowly and the body of the shot to be made tough.

These shot proved very effective against wrought-iron armour, but were not serviceable against compound and steel armour. A new departure had, therefore, to be made, and forged steel shot with points hardened by water took the place of the Palliser shot. At first these forged steel shot were made of ordinary carbon steel, but as armour improved in quality, the projectiles followed suit. For the attack of the latest type of cemented steel armour, the projectile is formed of steel - forged or cast - containing both nickel and chromium.

[edit] First World War Era

Shot and shell used prior to, and during World War I were generally cast from special chromium steel, that was melted in pots. They were afterwards forged into shape and then thoroughly annealed, the core bored at the rear and the exterior turned up in the lathe. The projectiles were finished in a similar manner to others described above. The final, or tempering treatment, which gave the require hardness/toughness profile (differential hardening) 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 an armour-piercing shell, not armour-piercing shot. The HE filling of the shell, whether fuzed or unfuzed, had a tendency to explode on striking armour in excess of its ability to perforate.

[edit] Second World War

During WWII, 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 APHE projectiles this could result in premature detonation of the HE filling. Highly advanced and precise methods of differentially hardening the projectile were developed during this period, especially by the German armament industry. The resulting projectiles gradual change from high hardness (low toughness) at the head, to a tough (less hard) rear and were much less likely to fail on impact.

Armour-piercing shells (APHE) for tank guns, although used by most forces of this period, were not used by the British. The only British APHE projectile was the Shell AP, Mk1 for the 2 pdr anti-tank gun and this was dropped as it was found the fuze tended to separate from the body during penetration. Even when the fuze didn’t separate and the system functioned correctly, damage to the interior was little different to the solid shot, and so did not warrant the addition time and cost of producing a shell version. APHE projectiles of this period used a bursting charge of about 1-3% of the weight of the complete projectile, the filling 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 know as ammonium picrate, for this purpose. Other combatant forces of the period used various explosives, suitability desensitised (usually by the use of waxes mixed with the explosive).

Due to the increase in armour thickness during the conflict, the projectiles’ impact velocity had to be increased to insure perforation. At these higher velocities the hardened tip of the shot or shell has to be protected by the initial impact shock, or risk shattering. To raise the impact velocity and stop the shattering they were initially fitted with soft steel penetrating caps. The best performance penetrating caps were not very aerodynamic, so an additional ballistic cap was later fitted to reduce drag. The resulting projectile types were given the names "Armour Piercing Capped" and "Armour Piercing Capped Ballistic Capped", the abbreviations for being APC and APCBC respectively.

Early WWII era uncapped AP projectiles fired from high-velocity guns were able to penetrate about twice their calibre at close range (100 m). At longer ranges (500-1,000 m) this dropped 1.5-1.1 calibres due to the poor ballistic shaped and higher drag of the smaller diameter early projectiles. Later in the conflict APCBC fired at close range (100 m) from large calibre, high-velocity guns (75-128 mm) were able to penetrate a much greater thickness of armour in relation to their calibre (2.5 times) and also a greater thickness (2-1.75 times) at longer ranges (1,500-2,000 m).

[edit] Modern Day

Although rarely encountered in large calibre tank guns now, armour-piercing shot and shell are used in the various medium calibre weapons (20-40 mm), most notably the automatic cannon used by the land, air and sea forces.

Armour-piercing shot for cannon tend to combine some form of incendiary capability with that of armour penetration. The incendiary compound is normally contained between the cap and penetrating nose, within a hollow at the rear, or a combination of both. If the projectile also uses a tracer, the rear cavity is often used to house the tracer compound. For larger calibre projectiles, the tracer may instead be contained within an extension of the rear sealing plug. Common abbreviations for solid (non-composite/hardcore) cannon fired shot are; AP, AP-T, API and API-T; where T stands for "tracer" and I for "incendiary"

Armour-piercing shells in the classic form are not common in modern cannon, though they may be found in the larger (40-57 mm) weapons, especially those of Russian, or Soviet era descent. Modern cannon instead fire semi-armour-piercing shells (SAPHE), these have less anti-armour capability, but far greater anti-material/personnel effects. The modern SAPHE projectiles still have a ballistic cap, hardened body and base fuze, but tend to have a far thinner body material and higher explosive content (4-15%). Common abbreviations for modern cannon AP and SAP shells are: HEI(BF), SAPHE, SAPHEI and SAPHEI-T.

It is interesting to note that a modern active protection system (APS) is unlikely to be able to defeat full calibre AP shot/shell fired from a large calibre tank gun. The APS can defeat the two most common anti-armour projectiles in use today: HEAT and APFSDS. The defeat of HEAT projectiles being accomplished through damage/detonation of the HE filling, damage to the shaped charge liner and/or fuzing system and for APFSDS projectiles by inducing yaw/pitch and/or fracturing of the rod. Due to the AP shot/shell's high mass, rigidity, short overall length and thick body, they are hardly affected by the defeat methods employed by APS systems (fragmentation warheads, or projected plates). Whether due to these potential failings, some form of large full calibre AP projectile will be introduced, or reinstated into service will be something to monitor.