The study identified 140 entry points for weapons on Brazilian borders.
"But the number of guns entering from land borders is negligible compared to the number of guns manufactured in the country, bought legally, but which then end up in the illegal market," said Antonio Rangel Bandeira, lead researcher for Viva Rio.
One of the causes for the rampant spread of arms in illegal markets is that policemen, firefighters and military personnel are allowed to acquire three guns annually at factory price. However, they often end up reselling those arms to supplement their income.
Barreto said Brazil would continue its disarmament campaign, which has encouraged people to return illegal weapons without asking questions where they came from.
The government launched disarmament campaigns in 2004, 2005, 2008 and 2009, collecting half a million weapons that were subsequently destroyed by the army.