An armor-piercing shell is a type of ammunition designed to penetrate armor. From the 1860s to 1950s, a major application of armor-piercing projectiles was to defeat the thick armor carried on many warships. From the 1920s onwards, armor-piercing weapons were required for anti-tank missions.
An armor-piercing shell must withstand the shock of punching through armor plating. Shells designed for this purpose have a greatly strengthened case with a specially hardened and shaped nose, and a much smaller bursting charge. Some smaller-caliber AP shells have an inert filling, or incendiary charge in place of the HE bursting charge. The AP shell is now little used in naval warfare, as modern warships have little or no armor protection, but it remains the preferred round in tank warfare, as it has a greater "first-hit kill" probability than a high explosive anti-tank (HEAT) round, especially against a target with composite armor, and because of higher muzzle velocity, is also more accurate than a HEAT round.
These chilled iron shots proved very effective against wrought-iron armor, but were not serviceable against compound and steel armor, 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 armor improved in quality, the projectiles followed suit.
During the 1890s and subsequently, cemented steel armor became commonplace, initially only on the thicker armor 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.
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 HE filling of the shell, whether fuzed or unfuzed, had a tendency to explode on striking armor in excess of its ability to perforate.
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 armor-piercing high-explosive (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 gradually change from high hardness (low toughness) at the head to high toughness (low hardness) 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 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 didn’t 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. 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 known 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 armor thickness during the conflict, the projectiles’ impact velocity had to be increased to ensure 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 "Armor-Piercing Capped (APC)" and "Armor-Piercing Capped Ballistic Capped (APCBC)".
Early WWII-era uncapped AP projectiles fired from high-velocity guns were able to penetrate about twice their caliber at close range (100 m). At longer ranges (500-1,000 m), this dropped 1.5-1.1 calibers due to the poor ballistic shape and higher drag of the smaller-diameter early projectiles. Later in the conflict, APCBC fired at close range (100 m) from large-caliber, high-velocity guns (75-128 mm) were able to penetrate a much greater thickness of armor in relation to their caliber (2.5 times) and also a greater thickness (2-1.75 times) at longer ranges (1,500-2,000 m).
It is interesting to note that a modern active protection system (APS) is unlikely to be able to defeat full-caliber AP rounds fired from a large-caliber tank gun. The APS can defeat the two most common anti-armor projectiles in use today: HEAT and APFSDS. The defeat of HEAT projectiles is accomplished through damage/detonation of the HE filling or damage to the shaped charge liner and/or fuzing system, and defeat of APFSDS projectiles is accomplished 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 some form of large full-caliber AP projectile will be introduced or reinstated into service due to these potential failings will be something to monitor.
Rifle and pistol rounds are usually built around a penetrator of hardened steel or tungsten. Aircraft and tank rounds sometimes use a core of depleted uranium. The penetrator is a pointed mass of high-density material that is designed to retain its shape and carry the maximum possible amount of energy as deeply as possible into the target. Depleted-uranium penetrators have the advantage of being pyrophoric and self-sharpening on impact, resulting in intense heat and energy focused on a minimal area of the target's armour. Some rounds also use explosive or incendiary tips to aid in the penetration of thicker armor.
Rifle armor-piercing ammunition generally carries its hardened penetrator within a copper or cupro-nickel jacket, similar to the jacket which would surround lead in a conventional projectile. Upon impact on a hard target, the copper case is destroyed, but the penetrator continues its motion and penetrates the target. Similar armor-piercing ammunition for pistols has also been developed and uses a similar design to the rifle ammunition.
The entire projectile is not normally made of the same material as the penetrator because the physical characteristics that make a good penetrator (i.e. extremely tough, hard metal) make the material equally harmful to the barrel of the gun firing the cartridge.
Contrary to common belief, Teflon or other coatings on a bullet do not in themselves help it penetrate more deeply. Teflon-coated bullets are designed purely to help reduce the wear on the barrel as a result of firing hardened projectiles. Teflon-coating bullets is a trend that has largely faded, in part because of laws resulting from this misconception.
|FN P80||.308||150 grain|