Following the First World War, tank armor was improved in terms of both thickness and quality. To be able to penetrate the armor of the tanks of potential enemies, it became necessary to design guns and shells for this specific purpose. During the thirties and fourties, a large number of new shell types saw combat. The descriptions below will give a brief overview of the main types of shells, explaining the basic designs, the method with which the shells penetrate an armor plate, and in which way they are designed to disable the tank against which they are fired.
Armor Piercing (AP)
The earliest and most simple anti-tank shell was the basic armor piercing, or AP, shell. An AP shell is made from solid steel with a high carbon contents, which increases the hardness of the steel.
Armor plate penetration is achieved by the kinetic energy of the shell. If the mass or the velocity of the shell is increased, so will the penetration capability of the shell. Inversely, as the velocity of the shell decreases over range due to air resistance, so will the penetration capability.
Assuming the shell penetrates the armor of the tank being fired at, if the shell doesn't contains an explosive filler, it will essentially act as a large bullet. It may damage the interior or the engine, cause the ammunition to explode, cause the fuel to catch fire, or injure the crew.
Armor Piercing, Capped (APC)
To increase the armor protection without increasing the armor plate's thickness, and consequently its weight, face hardening the armor plates was common during the Second World War. As the name implies, the face hardening process increases the hardness of the part of the armor plate facing outwards. This increased hardness will make it more difficult for the shell to penetrate the plate, and might even cause it to scatter on impact.
A countermeasure against face hardened plates is to place a cap on a regular armor piercing shell. This cap has a very hard tip, designed to break the hardened face, and a soft steel body, designed to protect the armor piercing shell from the force of impact.
While the actual penetration of the armor piercing shell is in itself the same as that of the uncapped armor piercing shell, the cap is a disadvantage when firing against regular armor plates that are not face hardened. The reason for this is that part of the mass, and therefore kinetic energy, of the shell is located in the cap, which does not aid the penetration of a regular armor plate.
Armor Piercing, Ballistic Cap (APBC)
Because the nose shape that is best suited for penetrating an armor plate is not the best in terms of aerodynamics, it was common to mount a ballistic cap, or windshield, on the shell. The ballistic cap is made from a thin, fragile material, that is destroyed on impact without interfering with the penetration process. Because the ballistic cap is thin, the negative impact from lack of mass is negligible, and more than offset by reduced decelleration due to the better aerodynamics.
An important note on ballistic caps is that the presence of ballistic caps is far from always explicit in American shell names. It is therefore necessary to find a description or schematic of a shell to determine the design.
Armor Piercing, Capped, Ballistic Cap (APCBC)
Explosive Filler (-HE)
To increase the lethality the above shell types, an explosive filler may be added to the shell. When the shell impacts the armor, a fuze is ignited, causing the shell to explode after penetrating. While the damage from the explosive filler can be substantial, the cavity in the shell in which the explosive filler is placed reduces the structural integrity of the shell. As a result, the shell is more likely to break up on impact, rather than penetrating.
Armor Piercing, Composite, Rigid (APCR)
While the velocity, and thus the kinetic energy, of a shell can in theory always be increased, there is a practical limit. While the kinetic energy is increased with the square of the speed increase, so is the amount of energy needed to propel the shell. An exponentially increasing amount of gun powder would make ammunition storage and loading cumbersome, and the increasing the canister size would require an expensive re-design and replacement of the gun breech, making existing ammunition incompatible with the gun.
An alternative to increasing the amount of gun powder is to decrease the caliber, and therefore the mass, of the penetrator. This small-caliber penetrator is made from a high-density material, such a Tungsten, and is housed in a shell made from a light-weight material, such as Aluminum, which has the same diameter as the gun barrel. The resulting shell is known as armor piercing, composite, rigid (APCR) in Europe, and high-velocity, armor piercing (HVAP) in the United States.
As the same amount of gun powder is spent on accelerating a smaller mass, the velocity is be increased. It is important to note that the increased velocity does not result in a higher kinetic energy. In fact, because the mass of the shell is smaller, it will decelerate more rapidly than a normal shell. Rather, the increased penetration of the APCR shell comes from the fact that the area being penetrated is smaller, increasing the amount of kinetic energy per square centimeter.
The APCR round is also more likely to richochet, and the smaller small will cause less damage than a full-caliber shell, especially since the penetrator does not contain an explosive filler. Nevertheless, during the Second World War, it allowed the service life of low-caliber guns designed before the war to be extended.
Armor Piercing, Composite, Non-Rigid (APCNR)
An alternative approach to the APCR sub-caliber penetrator is the armor piercing, composite, non-rigid (APCNR) shell. This shell type use the same principle as the APCR, but used the light-weight outer shell in a more active way.
Two different gun types fire this type of ammunition; Gerlich-style tapered, or squeeze, bores, where the gun is only designed to fire this type of ammunition, and regular guns fitted with the tapered Littlejohn adaptor. Common for both gun types is that the diameter of the barrel decreases towards the muzzle. This causes the light-weight outer part of the shell to tightly fit the gun barrel, preventing any gasses from escaping the barrel.
This gun design was mostly used by the Germans, most famously with the Schwere Panzer-Büchse 41, firing 28 mm shells that were reduced to 20 mm, but large-caliber guns were also designed, such as the 7,5 cm Pak 41, firing a 75 mm shell that was reduced to 55 mm.
The tapered bore design was ultimately a dead end. The wear on the gun barrel was excessive; for example, the 7,5 cm Pak 41 had a barrel life of 1000 rounds, compared to 5000-7000 rounds for the 7,5 cm Pak 39 (L/48). The problem with increased deceleration of the APCR shell is also present in the APCNR shell, making regular guns more effective at long ranges. In addition, while high-explosive shells did exist, the smaller end caliber meant that the explosive contents was similarly limited. Finally, the gun required large quantities of Tungsten, which for Germany was a scarce material.
Armor Piercing, Discarding Sabot (APDS)
The principle of concentrating the kinetic energy of a large-caliber shell in a narrow penetrator is taken to the extreme with the armor piercing, discarding sabot (APDS) shell. Developed during the war, this shell type is similar to the APCR in penetration principle. Rather than fixing the outer shell to the penetrator, the outer shell, or sabot, is discarded from the penetrator immediately upon leaving the muzzle. The penetrator itself is a long, thin rod of a high-density material, such as Tungsten, or, after fission energy became common efter the war, depleted Uranium.
APDS shells offer very good penetration capability, but also suffer from the same issues as APCR shells. APDS also have the disadvantage that the sabot will impact the ground in front of the gun at a relatively high velocity, posing a risk to friendly troops.
High Explosive, Anti-Tank (HEAT)
Unlike the above shells, all of which use kinetic energy to penetrate armor plates, the high explosive, anti-tank (HEAT) shell is a shaped charge. The shell is designed as a conical cavity with a copper lining, behind which is placed an explosive charge. When the shell hits its target, the explosive will cause the copper lining to form a stream of particles, which penetrates the armor plate at hyper-sonic speeds. This stream of particles will spray inside the tank, along with molten steel from the armor plate.
The main advantage of the HEAT shell over kinetic energy penetrators is that it does not depend on velocity. As a result, the HEAT shell is particularly well suited for infantry weapons, such as rifle grenades and rocket launchers. The Bazooka, PIAT, Panzerfaust, and Panzerschreck all fired HEAT shells. The disadvantages of the shell is that even a thin steel plate placed some distance from the armor plate will cause the particle stream to partially or fully dissipate without penetrating the armor. This was used on Russian tanks, where frames with fire nets were welded to turrets sides in the late stages of the war, as protection against the Panzerschreck and Panzerfaust. In addition, when fired from a rifled weapon, the rotation of the shell will decrease the penetration. Finally, the lower velocity of HEAT shells makes aiming at long ranges more difficult, and the shells were therefore unpopular with German anti-tank crews, as expressed in this report from 1943.
High Explosive (HE)
Traditionally, high-explosive shells have not been viewed as a significant threat to tanks. Common perception has been that, while artillery fire might damage tracks and exterior components, it is of little threat to the tank itself.
In 1988, the US Army conducted a test to see how effective 155 mm artillery was against Soviet tanks. During the test, it was found that the explosive force could immobilize the tanks, by destroying it tracks and tearing off road wheels, at a distance of up to 30 meters. Near hits would cause significant damage to the tanks' armor, while direct hits would completely destroy them. This is consistent with accounts from the Second World War where SU-152 would knock off the turret of German tanks. Lighter artillery and mortar fire could also cause significant damage to tanks, by hitting the thinner armor of the roof and engine deck.
Even if the armor is not penetrated, the concussion can cause fragments from the armor plates to injure the crew, and damage internal components. This effect was used after the war to design the high explosive squash head (HESH) shell, which squash a piece of plastic explosive onto the armor plate and then detonated it, causing a piece of the armor plate on the inside of the tank to seperate and damage the interior and crew, without actually penetrating the armor plate.
- Add-On Armor
- Methods used during the Second World War to improve the protection of armored vehicles.
- Armor Penetration Table
- Penetration table of German tank and anti-tank guns.
- DURHAM, Major (Retired) George A. Who Says Dumb Artillery Rounds Can't Kill Armor. Fort Sill, OK : US Field Artillery Association, 2002. 4 p.