By Laura MacInnis

GENEVA, Aug 28 (Reuters) - The United States has 90 guns for every 100 citizens, making it the most heavily armed society in the world, a report released on Tuesday said.

U.S. citizens own 270 million of the world's 875 million known firearms, according to the Small Arms Survey 2007 by the Geneva-based Graduate Institute of International Studies.

About 4.5 million of the 8 million new guns manufactured worldwide each year are purchased in the United States, it said.

"There is roughly one firearm for every seven people worldwide. Without the United States, though, this drops to about one firearm per 10 people," it said.

India had the world's second-largest civilian gun arsenal, with an estimated 46 million firearms outside law enforcement and the military, though this represented just four guns per 100 people there. China, ranked third with 40 million privately held guns, had 3 firearms per 100 people.

Germany, France, Pakistan, Mexico, Brazil and Russia were next in the ranking of country's overall civilian gun arsenals.

On a per-capita basis, Yemen had the second most heavily armed citizenry behind the United States, with 61 guns per 100 people, followed by Finland with 56, Switzerland with 46, Iraq with 39 and Serbia with 38.

France, Canada, Sweden, Austria and Germany were next, each with about 30 guns per 100 people, while many poorer countries often associated with violence ranked much lower. Nigeria, for instance, had just one gun per 100 people.

"Firearms are very unevenly distributed around the world. The image we have of certain regions such as Africa or Latin America being awash with weapons -- these images are certainly misleading," Small Arms Survey director Keith Krause said.

"Weapons ownership may be correlated with rising levels of wealth, and that means we need to think about future demand in parts of the world where economic growth is giving people larger disposable income," he told a Geneva news conference.

The report, which relied on government data, surveys and media reports to estimate the size of world arsenals, estimated there were 650 million civilian firearms worldwide, and 225 million held by law enforcement and military forces.

Five years ago, the Small Arms Survey had estimated there were a total of just 640 million firearms globally.

"Civilian holdings of weapons worldwide are much larger than we previously believed," Krause said, attributing the increase largely to better research and more data on weapon distribution networks.

Only about 12 percent of civilian weapons are thought to be registered with authorities.

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