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A mortar is an indirect fire weapon that fires explosive projectiles known as (mortar) bombs at low velocities, short ranges and high-arcing ballistic trajectories. It is typically muzzle-loading with short barrel, generally less than 15 times its caliber.
A mortar is relatively simple and easy to operate. A modern mortar consists of a tube into which assistant gunners drop a purpose-designed bomb. The tube is generally set at between 45 and 85 degrees angle to the ground, with the higher angle giving shorter firing distances. The bomb has a small baseline charge and no cartridge case; for extra range propellant rings are attached to the bomb's fins. When it reaches the base of the tube it hits a fixed firing pin, which detonates the baseline charge and fires the projectile. Some mortars have a moving firing pin, operated by a lanyard; others may be fired by a trigger.
These attributes contrast with the mortar's larger siblings, howitzers and field guns, that fire shells at higher velocities, longer ranges, flatter arcs, sometimes using direct fire. These weapons are also breech-loaded, while most mortars are muzzle-loaded.
From the 17th to the early 20th century very heavy, relatively immobile siege mortars were used, of up to one metre calibre, often made of cast iron and with outside barrel diameter many times that of the bore diameter. An early example was Roaring Meg used during The English Civil War. Smaller and more portable designs were introduced during the First World War, primarily for trench warfare, which took place at relatively close ranges. Mortars continue to be in use to the present day.
Light and medium mortars are portable, and usually used by infantry units. The chief advantage a mortar section has over an artillery battery is the flexibility of small numbers, mobility and the ability to engage targets in cover with plunging fire. Mortars are able to fire from the protection of a trench or other type of cover. In these aspects, the mortar is an excellent infantry support weapon, as it can be transported over any terrain and is not burdened by the logistical support needed for artillery.
Heavy mortars are typically between 120- and 300-mm calibre. These weapons are usually towed or vehicle-mounted, sometimes breech-loaded, and normally employed by infantry units attached to battalion through division level. Even at this size, mortars are simpler and less expensive than comparable howitzers or field guns.
A mortar can be carried by one or more people (larger mortars can usually be broken down into components), or transported in a vehicle. An infantry mortar can usually also be mounted and fired from a mortar-carrier, a purpose-built or modified armoured vehicle with a large roof-hatch. A mortar can also be a launcher for fireworks, a hand-held or vehicle-mounted projector for smoke shells or flares, or a large grenade launcher. Heavy mortars can be mounted on a towed carriage, or permanently vehicle-mounted as a self-propelled mortar. Twin-barrelled self-loading mortars—such as the Patria AMOS PT1—are the latest evolution of these heavy mortars and are mounted on platforms such as armoured personnel carriers, tank chassis, and coastal patrol boats.
Most modern mortar systems consist of three main components: a barrel, a base plate, and a bipod.
Modern mortars normally range in calibre from 60 mm (2.36 in) to 120 mm (4.72 in). However, mortars both larger and smaller than these specifications have been produced. An example of the smaller scale is the British 51 mm Light Mortar which is carried by an individual and consists of only a tube and a base plate. Conversely, a large example is the Soviet 2S4 M1975 Tyulpan (tulip flower) 240 mm self-propelled mortar.
Smaller mortars (up to 82 mm) are commonly used and transported by infantry based mortar sections as a substitute for, or in addition to, artillery.
Ammunition for mortars generally comes in two main varieties: fin-stabilised and spin-stabilised. The former have short fins on their posterior portion which control the path of the bomb in flight. Spin-stabilized mortar bombs rotate as they travel along and leave the mortar tube, which stabilizes them in much the same way as a rifle bullet. Both types of rounds can be either illumination (infra-red or visible illumination), smoke, or high explosive.
Spin-stabilised rounds may be fired from a smoothbore or a rifled barrel. Since mortars are generally muzzle loaded, mortar bombs for rifled barrels have a pre-engraved band, called an obturator, that engages with the rifling of the barrel. They are more accurate, but slower to load. Exceptions to this were the U.S. M2 4.2 inch mortar and M30 mortar, whose ammunition had a sub-caliber expandable ring that enlarges when fired. This allows the projectile to slide down the barrel freeling, but grip the rifling when fired. The system is reminiscent of the Minié ball for muzzle-loading rifles.
Mortars are made in a range of calibres. The French 81 mm mortar became standard for many countries, while the Soviet bloc standardized on the 82mm mortar.
Mortars suffer from instability when used on snow and soft ground, because the recoil pushes them into the ground or snow unevenly. A solution to this problem is the Raschen Bag.
Mortars and their ammunition are generally much smaller and lighter than other artillery, such as guns and howitzers, which allows light and medium (typically, 60mm and 81/82mm) mortars to be considered small-arms; i.e. capable of transport by personnel without vehicle assistance. They are short-range weapons, and often more effective than other artillery for many purposes within their shorter range. In particular, due its high, parabolic trajectory with a near vertical descent, the mortar can land bombs on nearby targets, including those behind obstacles or in fortifications, such as light vehicles behind hills or structures, or infantry in trenches or spider holes. This also makes it possible to launch attacks from positions lower than the target of the attack. (E.g., long-range artillery could not shell a target 1 km away and 30 metres (100 ft) higher; a target easily accessible to a mortar.
Mortars are also highly effective when used from concealed positions, such as the natural escarpments on hillsides or from woods, especially if forward observers are being employed in strategic positions to direct fire; an arrangement where the mortar is in relatively close proximity both to its FO and its target, allowing for fire to be quickly and accurately delivered to lethal effect.
Fin-stabilised mortar bombs do not have to withstand the rotational forces placed upon them by rifling or greater pressures, and can therefore carry a higher payload in a thinner skin than rifled artillery ammunition. Due to the difference in available volume, a smooth-bore mortar of a given diameter will have a greater explosive yield than a similarly sized artillery shell. For example, a 120 mm mortar bomb has about the same explosive capability as a 155 mm artillery shell. Also, fin-stabilised munitions fired from a smooth-bore, which do not rely upon the spin imparted by a rifled-bore for greater accuracy, do not have the drawback of veering in the direction of the spin.
Spigot mortars, a particular type of mortar, consist of a mostly solid rod or spigot, onto which a hollow tube in the projectile fits - inverting the normal tube-mortar arrangement. At the top of the tube in the projectile, a cavity contains propellant such as cordite. There is usually a trigger mechanism built into the base of the spigot, with a long firing pin running up the length of the spigot activating a primer inside the projectile and firing the propellant charge.
The advantage of a spigot mortar is that the firing unit (baseplate and spigot) is smaller and lighter than a conventional tube mortar of equivalent payload and range. It is also somewhat simpler to manufacture. Further, most spigot mortars have no barrel in the conventional sense, which means ammunition of almost any weight and diameter can be fired from the same mortar.
The disadvantage is that while most mortar bombs have a streamlined shape towards the back that fits a spigot mortar application well, using that space for the spigot mortar tube takes volume and mass away from the payload of the projectile. If a soldier is carrying only a few projectiles, the projectile weight disadvantage is not significant. However, the weight of a large quantity of the heavier and more complex spigot projectiles offsets the weight saved due to the spigot mortar being lighter than a conventional mortar.
A near-silent mortar can operate using the spigot principle. Each round has a close-fitting movable plug in the tube that fits over the spigot. When the round is fired, the projectile is pushed off the spigot, but before the plug clears the spigot it is caught by a constriction at the base of the tube. This traps the gases from the propelling charge and hence the sound of the firing. After World War II the Belgium Fly-K silent spigot mortar was accepted into French service as the TN-8111.
Spigot mortars are generally out of favor in modern usage, replaced by small conventional mortars.
Military applications of spigot mortars include
Non-military applications include the use of small-calibre spigot mortars to launch lightweight, low-velocity foam dummy targets used for training retriever dogs for bird hunting. Extremely simple launchers use a separate small primer cap as the sole propellant (similar or identical to the cartridges used in industrial nail guns).
Mortars have existed for hundreds of years, first seeing use in siege warfare. Many historians claim that the first mortars were used at the 1453 siege of Constantinople by Fatih Sultan Mehmed. A European account of the 1456 siege of Belgrade by Giovanni da Tagliacozzo credits the Ottoman Turks for using seven mortars that fired "stone shots one Italian mile high". The speed of these was apparently slow enough that casualties could be avoided by posting observers that gave warning of their trajectories.
Early mortars, such as the Pumhart von Steyr, were also large and heavy, and could not be easily transported. Simply made, these weapons were no more than iron bowls reminiscent of the kitchen and apothecary mortars from where they drew their name. An early transportable mortar was invented by Baron Menno van Coehoorn (Siege of Grave, 1673). An early use of these more mobile mortars as field (rather than siege) weapons was by the Government forces which suppressed the 1719 Jacobite rising at the Battle of Glen Shiel; they would have had great advantages over field guns in the almost roadless West Highlands of Scotland. Coehorn-type mortars of approximately 180 pounds (82 kg) weight were used by both sides during the American Civil War. At the Siege of Vicksburg, General US Grant reported making such mortars "by taking logs of the toughest wood that could be found, boring them out for six or twelve-pound shells and binding them with stong iron bands. These answered as coehorns, and shells were successfully thrown from them into the trenches of the enemy." 
During the Russo-Japanese War, Leonid Gobyato for the first time applied deflection from closed firing positions in the field and with General Roman Kondratenko designed the first mortar that fired navy shells. However, it was not until the Stokes trench mortar devised by Sir Wilfred Stokes in 1915, that the modern mortar transportable by one person was born. The Germans also developed a series of trench mortars or Minenwerfer in calibers from 7.58 cm to 25 cm during World War I, though these were rifled.
Extremely useful in the muddy trenches of the Western Front, mortars were praised because of the bombs' high angle of flight; a mortar round could be aimed to fall directly into trenches where artillery shells, due to their low angle of flight, could not possibly go. Modern mortars have improved upon these designs, offering a weapon that is light, adaptable, easy to operate, and yet possesses enough accuracy and firepower to provide the infantry with quality close fire support against soft and hard targets more quickly than any other means.
The largest mortars ever developed were the French "Monster Mortar" (36 French inches; 975 mm; developed by Henri-Joseph Paixhans in 1832), Mallet's Mortar (36 inches; 910 mm; designed by Robert Mallet and tested by the Woolwich Arsenal, London, in 1857) and the "Little David" (36 inches; 914.4 mm; developed in the United States for use in World War II). All three mortars had a caliber of 36 "inches", but only the "Monster Mortar" saw action (at the Battle of Antwerp in 1832).
Improvised, or "home-made", mortars have been used by insurgent groups, usually to attack fortified military installations or to terrorize civilians. The Provisional Irish Republican Army used some of the best-known examples during the 1970s, 1980s and 1990s. The largest types came to be known as "barracks busters" and were usually constructed from heavy steel piping mounted on a steel frame. The largest had a calibre of 320 millimetres (13 in) and fired home-made rounds carrying from 80 to 100 kilograms (180 to 220 lb) of explosive.
As each tube fired only one round, mortars were usually deployed as a battery of four or six welded onto a steel frame. This was often concealed inside a van such as a Ford Transit. The vehicle would be parked, pointing roughly at the target. A timer fired the propellant charges after a delay – this allowed the mortar gunner time to escape. After firing, a timer-operated incendiary device could set the vehicle on fire in order to destroy any forensic evidence it contained.
Well-known incidents using these weapons include the 1985 Newry mortar attack, when nine members of the Royal Ulster Constabulary were killed, and the Downing Street mortar attack in 1991. The IRA mortared 10 Downing Street as a Cabinet meeting was in session. Three bombs were launched but only one detonated, it landed in the back garden of the British Prime Minister's residence and only shattered the rear windows. Prime Minister John Major had to move to Admiralty House while repairs were effected.
French 120 mm MO-120-RT-61 mortar
81 mm high explosive mortar bomb fitted with contact fuze
Wiesel 2 lePzMrs (Lightweight Armoured Mortar of Advanced Mortar System) of the German Army
A Canadian mortar team in action in France, 1944.
Finnish mortar squad ready to fire with 81 KRH 71 Y
Australian 9.45 inch mortar in a concealed cellar near Lens, 30 January 1918
Handheld 60mm mortar in Afghanistan, 2010
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