A mortar is a muzzle-loading indirect fire weapon that fires shells at low velocities, short ranges, and high-arcing ballistic trajectories. It typically has a barrel length less than 15 times its caliber.
A mortar is relatively simple and easy to operate. A modern mortar consists of a tube which gunners drop a shell into. A firing pin at the base of the tube detonates the propellant and fires the shell.
Light and medium mortars are portable, and usually used by infantry units. The chief advantage a mortar section has over an artillery battery is its small numbers, mobility and the ability to engage targets in the defilade with plunging fires. It is able to fire from the protection of a trench or defilade. 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.
There are also heavy mortars of 120mm to 300mm caliber. 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 men (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 heavy mortar can be mounted on a towed carriage, or permanently vehicle-mounted as a self-propelled mortar. Twin-barreled self-loading mortars — such as the Patria AMOS PT1 — are the latest evolution of these heavy mortars and are mounted on platforms such as armored personnel carriers, tank chassis, and coastal patrol boats.
Modern mortars normally range in caliber from 60mm (2.36 in) to 120mm (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) 240mm self-propelled mortar.
Ammunition for mortars generally come in two main varieties: fin-stabilised and spin-stabilised. The former have short fins on their posterior portion which control the path of the shell in flight. Spin-stabilized mortars rotate as they travel along and leave the mortar tube. This action stabilizes them in much the same way that a rifle bullet is, or that the American football is stabilized when thrown in a "spiral." Both types of rounds can be either illumination (infra-red or visible illumination), smoke, or high explosive.
Spin-stabilised rounds require a rifled barrel. Since mortars generally are muzzle loaded, the mortar shell has a pre-engraved band, called an obturator, that engages with the rifling of the barrel. The increase in accuracy is at a cost in loading time and having to account for drift, a peculiarity of rifled systems that causes the round to "drift" perpendicular to the spin axis; this Magnus effect is common to any spinning projectile, and is, for example, what makes it possible for pitchers to throw curve balls.
One of the advantages in the use of mortars is the volume efficiency of their casing. While this doesn't hold true for rifled mortars, the smooth-bore models (as they do not have to withstand the rotational forces placed upon them by rifling) can be designed with much thinner skins, increasing the explosive load they can carry. 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. An excellent example is the comparison of a 120mm mortar and a 155mm artillery shell which have almost identical explosive powers.
Mortars come in a variety of calibers. The French 81mm mortar became standard for many countries. The Soviets took tactical advantage of this. They standardised an 82mm mortar for their armies. Hence, troops using Soviet mortars could use mortar ammunition of other countries found on the battlefield, albeit with less accuracy, while their own would be too large for their opponents. This advantage was used during the Vietnam War and at other times.
The advantage of a spigot mortar is that the firing unit (baseplate and spigot) is smaller and lighter than a conventional mortar of equivalent payload and range. It is also somewhat simpler to manufacture.
The disadvantage is that the mortar projectile requires additional material to contain the propellant gases during firing. While most mortar shells have a streamlined shape towards the back that naturally fits a spigot mortar application well, using that space for the spigot mortar tube takes volume and mass away from the explosive warhead payload and fragmentation mass 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 be made using the spigot principle. Each round has a closely fit but moveable 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, the plug is caught by a narrowing at the base of the tube. This traps the gases from the propelling charge and hence the sound of the firing. Post World War II the silent Belgium Fly-K 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
Nonmilitary applications include use of small-caliber 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 nailguns).
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 7.58cm to 25cm during World War I, though these were rifled.
Extremely useful in the muddy trenches of the Western Front, mortars were praised because of the shell's 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 mortars were usually deployed as a battery of four or six welded onto the same steel frame. The idea was that the improvised propellant fuses could be set once the mortar carrier was aimed roughly at the target and the mortars would automatically fire after a short delay. This allowed the mortar gunner to escape even before the mortar is fired.
The most famous use of this weapon was an IRA assassination attempt on 7 February 1991. The terrorists mortared 10 Downing Street as a Cabinet meeting was in session, however the bomb landed in the back garden of the British Prime Minister's residence only succeeding in shattering the back windows. Prime Minister John Major was forced to move to Admiralty House while repairs were effected.
Currently, mortars are being used by Palestinian guerrillas in the Gaza Strip against Israeli targets, including towns in Israel near the border with Gaza. A partial list of these attacks can be found in the List of Qassam rocket attacks article.