A howitzer is an artillery piece of the cannon family, with a ratio of caliber to barrel length intermediate between mortars and artillery guns, which delivers shells at medium velocity. While normally used for indirect fire, many howitzers can be used in direct fire, typically in final defense of the howitzer position.
Modern howitzers are usually self-propelled, although some towed howitzers are used in applications where weight is critical, such as when the howitzer must be delivered by parachute, amphibious landing vehicle, or helicopter.
For firing, howitzers are most often grouped into batteries of six pieces.
The 155mm howitzer is the most common in current use has a caliber of 155mm, although Soviet-designs tended to use 152mm but even Russia is moving to 155. 105mm howitzers are still used, especially by light forces delivered by helicopter or parachute. They have rifled barrels, a recoil mechanism, and can have their vertical (elevation) and horizontal (azimuth) aim points changed without moving the entire weapon.
Howitzers are frequently self-propelled, on a tracked chassis similar to that of a tank (e.g., U.S. M109 howitzer). Again, when air or ampbibious transportation requirements make light weight a requirement, the piece may be towed (e.g., U.S. M198, South African G5). Some light howitzers are being mounted in wheeled vehicles lighter than a tracked chassis.
An extremely lightweight 155mm howitzer, making extensive use of titanium construction, was first built by the British Vickers Group, and has been adopted as the U.S. M777, also used in Canada and being actively sold both by Britain, and BaE Systems that manufacture it in the U.S. A simultaneous breakthrough is the widespread deployment of an affordable, (USD $100,000) by military standards, guided shell for the M777, the XM982 Excalibur. The Excalibur is a joint development of the U.S. and Swedish armies. It is capable of Multiple Round Simultaneous Impact, firing several shells in different trajectories, such that they hit simultaneously.
German PzH howitzers, made by Krauss-Maffei Wegmann, also have an good reputation. The company has repackaged the weapon proper so it is no longer limited to its original tracked, self-propelled mount, but is a module that can go onto a variety of tracked or wheeled vehicles. It has been, for example, put onto the chassis of the M270 Multiple Launch Rocket System.
It has been relatively recent that heavy howitzers, such as the U.S. M110 8-inch/203mm, have disappeared from active military use. While this was long considered the most accurate cannon in the U.S. Army, rockets and missiles have greater range, accuracy and payload for a given weight. Air-dropped bombs also competed for the delivery of large amounts of explosive. Precision guidance also lets a smaller weight of explosive be more effective.
Even larger howitzers have been used, especially before the availability of aircraft and rocket-delivered weapons. A 9.2"/240mm howitzer saw field use in the First and Second World Wars.
Superheavy howitzers were not as mobile as field howitzers, but were appropriate for attacking fortified positions. The term "siege howitzer" goes back before the 20th century.