A mortar is an indirect fire device that launches projectiles at ranges from 70 meters to 7,200 meters. The mortar has traditionally been used as a weapon to propel explosive shells called mortar rounds in high-arcing ballistic trajectories. The weapon is typically muzzle-loading with a short, often smooth-bore barrel, generally less than 15 times its caliber. Modern mortars are light and easily portable. They can be used for close fire support with a variety of ammunition.
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-stabilized and spin-stabilized. Examples of 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. There are also training rounds and precision-guided rounds, such as the Strix mortar round. Mortar bombs are often referred to, incorrectly, as “mortars”
Operators may fire spin-stabilized rounds from either a smoothbore or a rifled barrel. Rifled-mortars are more accurate, but slower to load. Since mortars are generally muzzle-loaded, mortar bombs for rifled barrels usually have a pre-engraved band, called an obturator, that engages with the rifling of the barrel. Exceptions to this were the U.S. M2 4.2 inch mortar and M30 mortar, whose ammunition had a sub-caliber expandable ring that enlarged when fired. This allows the projectile to slide down the barrel freely, but grip the rifling when fired. The system resembles 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 or soft ground, because the recoil pushes them into the ground or snow unevenly. A Raschen bag addresses this problem.