English Civil War Artillery 1642–51

The mortar was the siege weapon par excellence … It could be used to throw explosive shells, incendiary devices and even showers of small stones into a town, with corresponding results. The mortar was normally cast from brass and was so designed that its powder chamber was of a smaller diameter than the bore of the weapon. Existing weapons in the Tower of London tend to have bore diameters of 18 inches but obviously mortars could be larger or smaller depending upon their use. They could also be cast with their trunnions at the mid-point of the gun or at one end. Originally this may have been because mortars were expected to fire over a wide range of angles of elevation. Eventually, when they became universally used to fire at elevations over 45 degrees, the trunnions were normally fitted to the base of the breech. Existing mortars seem mostly to have had trunnions in the middle in the 17th century. The mortar’s key advantage was that it could fire on a high trajectory and was therefore an indirect fire weapon, used to fire projectiles high over a town or castle wall to explode among the buildings behind.