The annual authorized trade in small arms has more than doubled in the past six years to $8.5 billion, driven by Americans buying more guns and ammunition and big military purchases for the wars in Iraq and Afghanistan, a survey found on Monday.

The Small Arms Survey 2012, an independent research project, found the top exporters with trade of at least $100 million were the United States, Italy, Germany, Brazil, Austria, Japan, Switzerland, Russia, France, South Korea, Belgium and Spain.

The top importers with trade of at least $100 million were the United States, Britain, Saudi Arabia, Australia, Canada, Germany and France, the survey found, and the most transparent were Switzerland, Britain and Romania.

While small arms trade transparency - public reporting on weapons commerce - improved by more than 40 percent between 2001 and 2010, the least transparent exporters were Iran, North Korea and the United Arab Emirates. The survey focused on government-approved transfers, not illicit trade.

“We think the authorized trade is larger than the illicit trade, although the illicit trade may do more damage or be more problematic,” Eric Berman, managing director of the Small Arms Survey, told a news conference in New York. “We can clearly say that the two combined would be over $10 billion.”

The survey found that for almost a decade the United States has been the top importer of small-caliber ammunition, sporting shotguns, pistols, revolvers and parts for small arms and light weapons, driven by civilian and military spending.

“Recreational hunters and other private individuals buy millions of imported rifles, shotguns and rounds of ammunition each year,” the survey said. “Millions of additional foreign-sourced weapons are procured by military and law enforcement agencies worldwide.”

The survey reviewed 10 years of reporting on the small arms trade by 52 countries exporting at least $10 million worth of such weapons.

The launch of the Small Arms Survey -- funded by Switzerland, Australia, Belgium, Canada, Denmark, Finland, Germany, the Netherlands, Norway, Sweden, Britain and the United States -- coincided with the start of the Second Review Conference of the U.N. Programme of Action on Small Arms and Light Weapons.