Artillery is a class of large military weapons built to fire munitions far beyond the range and power of infantrys small arms. Early artillery development focused on the ability to breach fortifications, and led to heavy, fairly immobile siege engines. As technology improved, lighter, more mobile field artillery developed for battlefield use. This development continues today; modern self-propelled artillery vehicles are highly mobile weapons of great versatility providing the largest share of an armys total firepower. In its earliest sense, the word artillery referred to any group of soldiers primarily armed with some form of manufactured weapon or armour. Since the introduction of gunpowder and cannon, the word "artillery" has largely meant cannon, and in contemporary usage, it usually refers to shell-firing guns, howitzers, mortars, rockets and guided missiles. In common speech, the word artillery is often used to refer to individual devices, along with their accessories and fittings, although these assemblages are more properly called "equipments". However, there is no generally recognised generic term for a gun, howitzer, mortar, and so forth: the United States uses "artillery piece", but most English-speaking armies use "gun" and "mortar". The projectiles fired are typically either "shot" (if solid) or "shell" (if not). "Shell" is a widely used generic term for a projectile, which is a component of munitions. By association, artillery may also refer to the arm of service that customarily operates such engines. In some armies one arm has operated field, coast, anti-aircraft artillery and some anti-tank artillery, in others these have been separate arms and in some nations coast has been a naval or marine responsibility. In the 20th Century technology based target acquisition devices, such as radar, and systems, such as sound ranging and flash spotting, emerged to acquire targets, primarily for artillery. These are usually operated by one or more of the artillery arms. The widespread adoption of indirect fire in the early 20th century introduced the need for specialist data for field artillery, notably survey and meteorological, in some armies provision of these are the responsibility of the artillery arm.