According to a CNN investigation in the United States around 270 million citizens own firearms, a figure that turns it in the most heavily armed country in the world.
Currently 40 states of the American union bear arms without permit or criteria reviews. Only nine states require requirements and tests before giving permission and only the state of Illinois does not lets you own firearms.
According to a Gallup survey for the United Nations, the United States has 88.8% of firearms per 100 people.
According to the Small Arms Survey, the massacres in the American Union are produced by the use of automatic and semi-automatic rifles, which are widely available, they are even sold in supermarkets. In some states only take a report of no criminal record in order to purchase a gun.