Mortars are artillery that fires at a steep trajectory, used originally in sieges to target buildings inside walled towns that would be unreachable by conventional artillery.
They have been used by armies ever since, though the modern mortar is a very different beast to its medieval counterpart. This pattern of man-portable mortar was developed in the Great War: when remaining in cover while targeting an enemy in cover was necessary to survive. This close-use artillery needed to be small enough to live in conventional trenches but provide greater firepower and range than rifle grenades.
The Stokes mortar was introduced by the British in 1915 and the design was widely modified and used by other countries. It is the grandfather of all modern mortars.
The Ordnance SMBL 2″
The 2″ mortar as used by the British Army in WW2 was developed from a Spanish 50mm mortar, though with modifications to make it suitable for British service. By the end of the war there had been eight marks with countless ‘*’ designations (used by the British at the time to denote small changes that did not add up to a full Mk).
Originally the mortar was supplied with a collimating sight with spirit-levels and adjustment to allow for carefully aimed fire, however this was dropped on many of the marks, being replaced by a simple white line up the length of the barrel which was pointed at the enemy and fire adjusted until the bombs landed on target.
Vintage Airsoft have built a prototype mortar and will be demonstrating it very soon.