A shell is a payload-carrying projectile, which, as opposed to shot, contains an explosive or other filling, though modern usage includes large solid projectiles previously termed shot (AP, APCR, APCNR, APDS, APFSDS and Proof shot). Solid shot may contain a pyrotechnic compound if a tracer or spotting charge is used.
Explosive shells in Europe do not appear to have been in general use before the middle of the 16th century, when hollow balls (i.e., hollow shells in the ordinary sense) of stone or cast iron were filled with gunpowder and a slow-burning composition to act as fuze and detonate the charge, and were fired from mortars. The flash from the discharge of the gun was supposed to ignite the fuze which would burn through and detonate the gunpowder after a certain time. In practice the flash did not always ignite the fuze, and the amount of fuze compound to burn for the right time before detonation could not be determined reliably.
Later shells were fitted with a hollow forged iron or copper plug, filled with slow-burning powder. While firing the gun might ignite the fuze, it was more reliable to ignite the fuze manually and then fire. This required that the muzzle-loading barrel be short enough for the gunner to readily reach in, ignite the fuze, and get out of the way before the gun fired. The short barrel produced low muzzle velocities which required high trajectories; the guns were mortars or howitzers.
In 1823, the first guns to fire explosive shells with the flat trajectory of cannons were invented by the French General Henri-Joseph Paixhans. The guns were adopted by various navies from the 1840s, thereby triggering the demise of wooden ships, and the iron-hull revolution in shipbuilding. Cast-iron spherical common shell (so named because they were used against "common" [usual] targets) were in use up to 1871. Towards the end of this period they were fitted with a wooden disc called a sabot, attached by a copper rivet, intended to keep the fuze centered when loading. The sabot was also supposed to reduce the rebounding tendency of the shell as it traveled along the bore on discharge. Mortar shells were not fitted with sabots.
Cast iron was used for projectiles until the end of the 19th century, when steel came into use, first for projectiles intended for piercing armor, and later for use in high-velocity guns where the shock of discharge was too severe for cast iron.
During the First World War, shrapnel from explosive shells inflicted terrible casualties on infantry, accounting for nearly 70% of all war casualties and leading to the adoption of steel helmets on both sides. Shells filled with poison gas were used from 1917 onwards. Frequent problems with shells led to many military disasters when shells failed to explode, most notably during the 1916 Battle of the Somme.
The calibre of a shell is its diameter. Depending on the historical period and national preferences, this may be specified in millimetres, centimetres, or inches. The length of gun barrels for large cartridges and shells (naval) is frequently quoted in terms of calibre.  Some guns, mainly British, were specified by the weight of their shells (see below).
Due to manufacturing difficulties the smallest shells commonly used are around 20 mm calibre, used in aircraft cannon and on armoured vehicles. Smaller shells are only rarely used as they are difficult to manufacture and can only have a small explosive charge. The largest shells ever fired were those from the German super-railway guns, Gustav and Dora, which were 800 mm (31.5") in calibre. Very large shells have been replaced by rockets, guided missile, and bombs, and today the largest shells in use are 203 mm (8 inches). Guns of that size are uncommon; 155 mm (6.1 inches) is the largest calibre in common use.
Gun calibres have standardized around a few common sizes, especially in the larger range, mainly due to the uniformity required for efficient military logistics. Shells of 105, 120, and 155 mm diameter are common for NATO forces' artillery and tank guns. Artillery shells of 122, 130 and 152 mm, and tank gun ammunition of 100, 115, or 125 mm calibre remain in use in Eastern Europe and China. Most common calibres have been in use for many years, since it is logistically complex to change the calibre of all guns and ammunition stores.
The weight of shells increases by and large with calibre. A typical 150 mm (5.9") shell weighs about 50 kg, a common 203 mm (8") shell about 100 kg, a concrete demolition 203 mm (8") shell 146 kg, a 280 mm (11") battleship shell about 300 kg, and a 460 mm (18") battleship shell over 1500 kg. The Schwerer Gustav supergun fired 4.8 and 7.1 tonne shells.
Early explosives used before and during World War I in HE shells were Lyddite (picric acid), PETN, TNT. Amatol was developed during WWI and was still in wide use in World War II, then RDX from 1945. Modern high explosives used include "Composition B" (cyclotol).
The earliest naval and anti-tank shells had to withstand the extreme shock of punching through armor plate. Shells designed for this purpose sometimes had a greatly strengthened case with a small bursting charge, and sometimes were solid metal, i.e. shot. In either case, they almost always had a specially hardened and shaped nose to facilitate penetration. A further refinement of such designs improved penetration by adding a softer metal cap to the penetrating nose giving APC (Armour piercing - capped). The softer cap dampens the initial shock that would otherwise shatter the round. The best profile for the cap is not the most aerodynamic; this can be remedied by adding a further hollow cap of suitable shape: APCBC (APC + Ballistic Cap). AP shells with a bursting charge were sometimes distinguished by appending the suffix "HE". At the beginning of the Second World War, solid shot AP projectiles were common. As the war progressed, ordnance design evolved so that APHE became the more common design approach for anti-tank shells of 75 mm caliber and larger, and more common in naval shell design as well. In modern ordnance, most full caliber AP shells are APHE designs.
APDS was developed by engineers working for the French Edgar Brandt company, and was fielded in two calibers (75 mm/57 mm for the Mle1897/33 75 mm anti-tank cannon, 37 mm/25 mm for several 37 mm gun types) just before the French-German armistice of 1940. The Edgar Brandt engineers, having been evacuated to the United Kingdom, joined ongoing APDS development efforts there, culminating in significant improvements to the concept and its realization. British APDS ordnance for their QF 6 pdr and 17 pdr anti-tank guns was fielded in March 1944.
The armor-piercing concept calls for more penetration capability than the target's armour thickness. Generally, the penetration capability of an armor piercing round is proportional to the projectile's kinetic energy. Thus an efficient means of achieving increased penetrating power is increased velocity for the projectile. However, projectile impact against armour at higher velocity causes greater levels of shock. Materials have characteristic maximum levels of shock capacity, beyond which they may shatter on impact. At relatively high impact velocities, steel is no longer an adequate material for armor piercing rounds due to shatter. Tungsten and tungsten alloys are suitable for use in even higher velocity armour piercing rounds due to their very high shock tolerance and shatter resistance. However, tungsten is very dense, and tungsten rounds of full-caliber design are too massive to be accelerated to an efficient velocity for maximized kinetic energy. This is overcome by using a reduced-diameter tungsten shot, surrounded by a lightweight outer carrier, the sabot (a French word for a wooden shoe). This combination allows the firing of a smaller diameter (thus lower mass/aerodynamic resistance/penetration resistance) projectile with a larger area of expanding-propellant "push", thus a greater propelling force/acceleration/resulting kinetic energy.
Once outside the barrel, the sabot is stripped off by a combination of centripetal force and aerodynamic force, giving the shot low drag in flight. For a given caliber the use of APDS ammunition can effectively double the anti-tank performance of a gun.
An Armour-Piercing, Fin-Stabilised, Discarding Sabot (APFSDS) projectile uses the sabot principle with fin (drag) stabilisation. A long, thin sub-projectile has increased sectional density and thus penetration potential. However, once a projectile has a length-to-diameter ratio greater than 10 (less for higher density projectiles), spin stabilisation becomes ineffective. Instead, drag stabilisation is used, by means of fins attached to the base of the sub-projectile, making it look like a large metal arrow.
Large calibre APFSDS projectiles are usually fired from smooth-bore (unrifled) barrels, though they can be and often are fired from rifled guns. This is especially true when fired from small to medium calibre weapon systems. APFSDS projectiles are usually made from high-density metal alloys such as tungsten heavy alloys (WHA) or depleted uranium (DU); maraging steel was used for some early Soviet projectiles. DU alloys have better penetration than others as they are denser and self-sharpening, but they present radiological and toxic hazards that remain on the battlefield. The less toxic WHAs are preferred in most countries except the USA, UK, and Russia.
HEAT shells are a type of shaped charge used to defeat armoured vehicles. They are extremely efficient at defeating plain steel armour but less so against later composite and reactive armour. The effectiveness of the shell is independent of its velocity, and hence the range: it 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 and detonating it at the correct distance, e.g., PIAT bomb. HEAT shells are less effective if spun (i.e., fired from a rifled gun).
HESH is another anti-tank shell based on the use of explosive. Developed by the British inventor Sir Charles Dennistoun Burney in World War II for use against fortifications. A thin-walled shell case contains a large charge of a plastic explosive. On impact the explosive flattens, without detonating, against the face of the armour, and is then detonated by the fuze. Energy is transferred through the armour plate: when the compressive shock reflects off the air/metal interface on the inner face of the armour, it is transformed into a tension wave which spalls a "scab" of metal off into the tank damaging the equipment and crew without actually penetrating the armour.
HESH is completely defeated by spaced armour, so long as the plates are individually able to withstand the explosion. It is still considered useful as not all vehicles are equipped with spaced armour, and it is also the most effective munition for demolishing brick and concrete. HESH shells, unlike HEAT shells, can be fired from rifled guns.
A proof shot is not used in combat but to confirm that a new gun barrel can withstand operational stresses. The proof shot is heavier than a normal shot or shell, and an oversize propelling charge is used, subjecting the barrel to greater than normal stress. The proof shot is inert (no explosive or functioning filling) and is often a solid unit, although water, sand or iron powder filled versions may be used for testing the gun mounting. Although the proof shot resembles a functioning shell (of whatever sort) so that it behaves as a real shell in the barrel, it is not aerodynamic as its job is over once it has left the muzzle of the gun. Consequently it travels a much shorter distance, and if stopped by an earth bank or similar the impact is less.
The gun, operated remotely for safety in case it fails, fires the proof shot, and is then inspected for damage. If the barrel passes the examination "proof marks" are added to the barrel. The gun can be expected to handle normal ammunition, which subjects it to less stress than the proof shot, without being damaged.
Shrapnel shells were an early (1784) anti-personnel munition which delivered large numbers of bullets at ranges far greater than rifles or machine guns could attain - up to 6,500 yards by 1914. A typical shrapnel shell as used in World War I was streamlined, 75 mm (3 inch) in diameter and contained approximately 300 lead-antimony balls (bullets), each approximately 1/2 inch in diameter. Shrapnel used the principle that the bullets encountered much less air resistance if they travelled most of their journey packed together in a single streamlined shell than they would if they travelled individually, and could hence attain a far greater range.
The gunner set the shell's fuze so that it was timed to burst as it was angling down towards the ground just before it reached its target (ideally about 150 yards before, and 60-100 feet above the ground). The fuze then ignited a small "bursting charge" in the base of the shell which fired the balls forward out of the front of the shell case, adding approximately 200 - 250 ft/second to the existing velocity of 1200 - 750 ft/second. The shell case dropped to the ground and the bullets continued in an expanding cone shape before striking the ground over an area approximately 250 yards x 30 yards in the case of the US 3 inch shell. The effect was of a large shotgun blast just in front of and above the target, and was deadly against troops in the open. A trained gun team could fire 20 such shells per minute, with a total of 6,000 balls, which compared very favourably with rifles and machine-guns.
However, shrapnel's relatively flat trajectory (it depended mainly on the shell's velocity for its lethality, and was only lethal in a forward direction) meant that it could not strike trained troops who avoided open spaces and instead used dead ground (dips), shelters, trenches, buildings, trees for cover. It was of no use in destroying buildings or shelters. Hence it was replaced during World War I by the high-explosive shell which exploded its fragments in all directions and could be fired by high-angle weapons such as howitzers, hence far more difficult to avoid.
Artillery-scattered mines allow for the quick deployment of minefields into the path of the enemy without placing engineering units at risk, but artillery delivery may lead to an irregular and unpredictable minefield with more unexploded ordnance than if mines were individually placed. Signatories of the Ottawa Treaty have renounced the use of artillery-scattered mines.
The fuze of a shell has to keep the shell safe from accidental functioning during storage, due to (possibly) rough handling, fire, etc, it also has to survive the violent launch through the barrel, then reliably function at the correct time. To do this it has a number of arming mechanisms, which are successively enabled under the influence of the firing sequence.
Sometimes, one or more of these arming mechanisms fails, and if the fuze is installed on an HE shell, it fails to detonate on impact. More worrying and potentially far more hazardous are fully armed shells on which the fuze fails to initiate the HE filling. This may be due to shallow, low velocity or soft impact conditions. Whatever the reason for failure, such a shell is called a blind or unexploded ordnance (UXO). The older term, "dud", is discouraged because it implies that the shell cannot detonate. Blind shells often litter old battlefields and depending on the impact velocity may be buried some distance into the earth, all remain potentially hazardous. For example, antitank ammunition with a piezoelectric fuze can be detonated by relatively light impact to the piezoelectric element, and others, depending on the type of fuze used can be detonated by even a small movement. The battlefields of the First World War still claim casualties today from leftover munitions. Thankfully modern electrical and mechanical fuzes are highly reliable, if they do not arm correctly they keep the initiation train out of line, or if electrical in nature, discharge any stored electrical energy.