Most shells are roughly bullet shaped - that is, a cylinder topped by an ogive shaped nose, possibly with a tapering base - but some specialised types are quite different.
Calibres
The calibre of a shell is its diameter. This is normally measured in millimetres even in the USA, although it is occasionally given in centimetres in some European countries, or even in inches for some naval guns.
The smallest shells are 20 mm calibre, used in aircraft cannon and some light armoured vehicles. The largest shells ever fired were those from the German super-railway guns, Gustav and Dora, which were 800 mm ( 31.5") in calibre.
Such large shells have been largely obsoleted by guided missiles, and today shells larger than 155 mm are rare.
Some common shell calibres (all in mm) include 20, 25, 30, 35, 40, 76, 84, 105, 120, 127, and 155.
Old style calibres
Historically, shells were often described in pounds. Usually this refers to the actual weight of an HE shell, but for maximum confusion this is not always the case. Some were named after the weights of obsolete shell types of the same calibre, or even obsolete shell types that were considered to have been functionally equivalent. Also non-HE shells fired from the same gun, but usually actually different in weight, were given the same poundage. Thus to convert from "pounds" to mm requires consulting a detailed historical reference.
Types
There are many different types of shells. The principle ones include:
High Explosive (HE)
The most common shell type is high explosive, commonly referred to simply as HE. HE shells have a strong steel case, a bursting charge, and a fuze. When the fuze initiates the shell, the bursting charge shatters the case and scatters hot, sharp fragments of steel at high speed. Most of the damage is caused by being struck by these fragments, rather than directly by the blast. The fuze on an HE shell can usually be set to burst on the ground, in the air above the ground (to scatter fragments behind cover), or after penetrating a short distance into the ground (either to transmit more ground shock to covered positions, or to reduce the spread of fragments).
Armour-piercing (AP)
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 had no bursting charge at all. Plain AP shell is now very rarely seen except in naval usage, and is less common even there. See also: Armor-piercing shot and shell
Armour-piercing, Discarding Sabot (APDS)
APDS extends the concept of AP by carrying a sub-calibre (i.e., smaller than the gun barrel) projectile in a light alloy "sabot" which fills up the rest of the barrel. (After emerging from the barrel, the sabot is stripped off by a combination of centripetal force and wind.) This enables the projectile to reach much higher velocities, increasing its penetrating power. To withstand the greater shock of impact, and increase density for greater penetration, the projectile is usually made from tungsten.
A further refinement of the same concept is Armour-piercing, Discarding Sabot, Fin Stabilised. In this the projectile is made long and thin to increase its sectional density and thus penetration. However once a projectile is more than about ten times longer than it is wide, spin stabilisation becomes ineffective, so the projectile is instead stabilised by fins attached at its base. Thus, an APFSDS projectile looks like a big metal arrow! APFSDS projectiles are sometimes made from tungsten, but the most effective types are made from depleted uranium.
High Explosive, Anti-Tank (HEAT)
HEAT shells are a type of shaped charge used to defeat armoured vehicles. They are extremely efficient at defeating plain steel armour but are becoming less useful with the growing prevalence of reactive armour. Because a HEAT charge is best detonated at a certain optimal distance in front of the target, HEAT shells are usually distinguished by a long, thin nose probe sticking out in front of the rest of the shell.
High Explosive, Squash Head (HESH) or High Explosive, Plastic (HEP)
A HESH shell is another anti-tank shell. It is has a very thin case, and an unusually large charge of a plastic explosive. It is designed to flatten against the face of the armour, and detonate at the time that transfers the maximum shock into the armour plate. When the compressive shock reflects off the air/metal interface inside the tank, it is transformed into a tension wave which spalls a "scab" of metal off the inside of the plate and throws it into the tank. Thus a HESH shell can defeat a tank even without penetrating the armour.
HESH is completely defeated by spaced armour (provided that the plates are individually able to withstand the explosion), but remains popular in some armed forces because vehicles without spaced armour are still common, and it is also the most efficient shell at demolishing brick and concrete.
Sometimes, one of these safety mechanisms is not disabled during the shell's flight, and the shell fails to detonate on impact. Such a shell is called a blind or UXO. (An older term, "dud", is discouraged because it implies that the shell cannot detonate). Blind shells often litter old battlefields (sometimes burrowed a short distance into the earth), and are extremely hazardous. For example, there is at least one type of blind which can be detonated by a shadow passing across it on a hot day, and most types can potentially be detonated by even a small movement.
If a blind shell is discovered, it should be avoided, other people warned of its presence, and it should be reported to the local police or armed forces for safe destruction.
Explosive shells do not appear to have been in general
use before the middle of the 16th century. About that
time hollow balls of stone or cast iron were fired from
mortars. The balls were nearly filled with gunpowder and the remaining space with a slow-burning composition. This method was fairly ineffective as the charge was not always ignited by the flash from the discharge of the gun, and moreover the amount of composition to burn a stipulated time could not easily be gauged.
The shell was, therefore, fitted with a hollow forged iron or copper plug, filled with slow-burning powder. It was impossible to ignite with certainty this primitive fuze simply by firing the gun; the fuze was consequently first ignited and the gun fired immediately afterwards. This entailed the use of a mortar or a very short piece, so that the fuze could be easily reached from the muzzle without unduly endangering the gunner. Cast-iron spherical
common shell were in use up to 1871. For guns they were latterly fitted with a wooden disc called a sabot, attached by a copper rivet, intended to keep the fuze central when loading. They were also supposed to reduce the rebounding tendency of the shell as it travelled along the bore on discharge. Mortar shell were not fitted with sabots.
Cast iron held its own as the most convenient material
for projectiles up to the end of the 19th century, steel supplanting it, first for projectiles intended for piercing armour, and afterwards for common shell for high-velocity guns where the shock of discharge has been found too severe for cast iron.Artillery delivered mines
A type of carrier shell which scatters landmines to create an instant minefield at a remote location. Signatories of the Ottawa treaty have renounced these shells.
Chemical
Chemical shells contain just a small explosive charge to burst the shell, and a larger quantity of a chemical weapon. Signatories of the Chemical Weapons Convention have renounced such shells.
Non-lethal shells
Not all shells are designed to kill or destroy. The following three types are designed to achive particular non-lethal effects on the battlefield. They are not completely harmless, however; smoke and illumination shells can accidentally start fires, while all three types can cause minor damage (or potentially kill) if property or a person is unlucky enough to be struck by the discarded carrrier.
Smoke
The smoke shell is designed to create a smoke screen. The main types are bursting (usually filled with white phosphorus) and base ejection (a particular type of carrier shell, which scatters specialised smoke grenades).
Illumination
Another non-lethal shell type is illumination. An illumination shell has a fuze which ejects the "candle" (a pyrotechnic flare emitting white, coloured, or infrared light) at a calculated altitude, where it slowly drifts down beneath a heat resistant parachute.
Carrier
The carrier shell is simply a hollow carrier equipped with a fuze which ejects the contents at a calculated time. They are often filled with propaganda leaflets, but can be filled with anything that meets the weight restrictions and is able to withstand the shock of firing.
Unexploded shells
The fuze of a shell has to keep the shell safe from accidental detonation during storage, (possibly rough) handling, and violent launch through the barrel, then reliably detonate it at the correct time. To do this it has a number of safety mechanisms which are successively withdrawn under the influence of the sequence of firing.History