A shell is a projectile, which, as opposed to a bullet, is not solid but contains an explosive or other filling.
These objects of weaponry are generally large rounds fired by artillery, armored fighting vehicles (including tanks), and warships, such as battleships.
Most shells are aerodynamic and hence, tend to have similar shapes to bullets—that is, a cylinder topped by an ogive shaped nose, possibly with a tapering base—but some specialised types are quite different.
The calibre of a shell is its diameter. Depending on the historical period and national preferences this may be given in millimetres, centimetres, or inches. Care should be taken as the length of gun barrels is frequently quoted in terms of Calibre.
Due to the problems of manufacture, the lower size limit for shells is a calibre around 20 mm, used in aircraft cannon and on armoured vehicles. Smaller explosive projectiles exist, but they are rare. The largest shells ever fired were those from the German super-railway guns, Gustav and Dora, which were 800 mm (31.5") in calibre. Large shells have been replaced by rockets, guided missile and bombs, and today the largest shells in use are 203 mm (8 inches), and guns of that size are uncommon; 155 mm (6 inches) is the largest caliber in common use.
Guns exist to fire most calibres in the 20 to 40 mm range, though at the larger end there is a degree of standardization around a few common sizes, mainly due to the uniformity required for efficient military logistics. Shells of 105 mm, 120 mm and 155 mm diameter are common for artillery and tank guns, and 122 mm and 152 mm calibers remain in use among Communist Bloc countries. Most common calibers have been in used for many years, since it is no small feat to change over ammunition stockpiles.
Old style calibres
Historically, shells were often described in pounds. Usually this refers to the actual weight of an HE shell, but, confusingly, this was 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 of different weight, were assigned the same poundage. Thus, conversion from "pounds" to actual barrel diameter requires consulting a historical reference.
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 fuse. When the fuse 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. Depending on the type of fuze used the HE shell can be set to burst on the ground, in the air above the ground, 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).
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 or even no bursting charge for smaller calibres.
A further refinement of the design improved penetration by adding a softer metal cap to the penetrating nose giving APC (Armour piercing - capped). The softer cap took away some of the initial shock that would otherwise shatter the round. However the best profile for the cap was not the best for flight. To restore aerodynamics a further hollow cap was added to give APCBC (APC + Ballistic Cap).
Explosive AP shells were sometimes distinguished by appending the suffix "-HE" or "/HE". Solid shot AP projectiles were so uncommon, that for unnecessary repetition the suffix "-HE" is usually not used; all projectiles can be assumed to have even small explosive charge. Plain AP shell is now very rarely seen except in naval usage, and is uncommon even there. See also: Armor-piercing shot and shell
Armour-piercing, Discarding Sabot (APDS)
APDS was developed by the United Kingdom and put into British service in March 1944 with their 6 pdr and 17 pdr anti-tank guns. For increased penetrating power a high velocity round was required, this in turn required a stronger material (such as tungsten) to withstand the greater shock of impact. Such a shot was too heavy at full bore to be accelerated to a sufficient muzzle velocity, so a lightweight outer carrier, the Sabot, (French shoe) which filled the barrel was fitted around the smaller-diameter shot. This gives the projectile a higher acceleration in the gun's barrel, due to the larger surface area for the gases to impinge upon relative to its weight. Once outside the barrel, the sabot is stripped off by a combination of centripetal force and wind, giving the shot low drag in flight. For a given caliber, this type of ammunition can effectively double the anti-tank performance of a gun over those using "simple" shot.
A further refinement of the same concept is the Armour-piercing, Fin Stabilised, Discarding Sabot. 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, and is fired from an unrifled barrel. 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.
APDS and APFSDS rounds are solid "shot" and contain no explosive charge and are not therefore "shells".
Armour-Piercing, Composite Rigid (APCR)
A variation on the APDS theme. A solid high-density metal core surrounded by a full bore shell of lighter material. In effect a discarding sabot that didn't discard.
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 composite and reactive armour. The power of the shell is independant of the velocity of the shell and is as effective at 1000 metres as at 100 metres. A HEAT charge is most effective when detonated at a certain, optimal, distance in front of the target and HEAT shells are usually distinguished by a long, thin nose probe sticking out in front of the rest of the shell, e.g., PIAT bomb.
High Explosive, Squash Head (HESH) or High Explosive, Plastic (HEP)
A HESH shell is another anti-tank shell also developed by the United Kingdom. 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.
Like cluster bombs, an artillery shell may be used to scatter smaller submunitions, including anti-personnel grenades, anti-tank top-attack munitions, and landmines. These are generally far more lethal against both armor and infantry than simple high explosive shells, since the multiple munitions create a larger kill zone and increase the chance of achieving the direct hit necessary to kill armor. Most modern armies make significant use of cluster munitions in their artillery batteries.
Artillery-scattered mines allow for the quick deployment of minefields into the path of the enemy without placing engineering units at risk, though artillery delivery may lead to an irregular and unpredictable minefield with more duds than if mines were individually emplaced. Signatories of the Ottawa Treaty have renounced the use of artillery-scattered mines.
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.
Not all shells are designed to kill or destroy. The following three types are designed to achieve 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.
The smoke shell is designed to create a smokescreen. The main types are bursting (usually filled with white phosphorus, WP) and base ejection (a shell which scatters smoke grenades).
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. These are also known as starshell.
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. Famously, on Christmas Day 1899 during the siege of Ladysmith, the Boers fired into Ladysmith a carrier shell without fuze, which contained a Christmas pudding, two Union Jacks and the message "compliments of the season". The shell is still kept in the museum at Ladysmith.
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.
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.