The arms industry is a global business that manufactures weapons and military technology and equipment. It consists of commercial industry involved in research, development, production, and the service of military material, equipment, and facilities. Arms producing companies, also referred to as defense contractors or military industry, produce arms mainly for the armed forces of states. Departments of government also operate in the arms industry, buying and selling weapons, munitions and other military items. Products include guns, ammunition, missiles, military aircraft, military vehicles, ships, electronic systems, and more. The arms industry also conducts significant research and development. It is estimated that yearly, over 1.5 trillion US dollars are spent on military expenditures worldwide (2.7% of World GDP). Part of this goes to the procurement of military hardware and services from the military industry. The combined arms sales of the top 100 largest arms producing companies amounts to an estimated $315 billion. Many industrialized countries have a domestic arms industry to supply their own military forces. Some countries also have a substantial legal or illegal domestic trade in weapons for use by its citizens. The Small Arms Survey estimates 875 million small arms in circulation worldwide, produced by more than 1,000 companies from nearly 100 countries.