Gun laying is the process of aiming an artillery piece, such as a gun, howitzer or mortar on land, or at sea, against surface or air targets. It may be laying for direct fire, where the gun is aimed similarly to a rifle, or indirect fire, the term includes automated aiming using, for example, radar-derived target data and computer-controlled guns. Gun laying means moving the axis of the bore of the barrel in two planes, horizontal and vertical. A gun is traversed – rotated in a horizontal plane – to align it with the target, Gun laying is a set of actions to align the axis of a gun barrel so that it points in the required direction. This alignment is in the horizontal and vertical planes, Gun laying may be for direct fire, where the layer sees the target, or indirect fire, where the target may not be visible from the gun. Gun laying has sometimes called training the gun. Laying in the vertical plane uses data derived from trials or empirical experience, for any given gun and projectile types, it reflects the distance to the target and the size of the propellant charge. It also incorporates any differences in height between gun and target, with indirect fire, it may allow for other variables as well. With indirect fire the horizontal angle is relative to something, typically the guns aiming point, depending on the gun mount, there is usually a choice of two trajectories. The dividing angle between the trajectories is about 45 degrees, it varies due to gun dependent factors. Below 45 degrees the trajectory is called low angle, above is high angle, the differences are that low angle fire has a shorter time of flight, a lower vertex and flatter angle of descent. All guns have carriages or mountings that support the barrel assembly, early guns could only be traversed by moving their entire carriage or mounting, and this lasted with heavy artillery into World War II. Mountings could be fitted into traversing turrets on ships, coast defences or tanks, from circa 1900 field artillery carriages provided traverse without moving the wheels and trail. The carriage, or mounting, also enabled the barrel to be set at the elevation angle. With some gun mounts it is possible to depress the gun, some guns require a near-horizontal elevation for loading. An essential capability for any elevation mechanism is to prevent the weight of the barrel forcing its heavier end downward and this is greatly helped by having trunnions at the centre of gravity, although a counterbalance mechanism can be used. It also means the elevation gear has to be enough to resist considerable downward pressure. However, mortars, where the forces were transferred directly into the ground
US Army self-propelled howitzer direct fire.
Manual traverse for an Eland armoured car. Gun elevation is controlled by the left traverse wheel, horizontal turret rotation by the right.