Hard to say.
If you break it down by country, Brazil exported more firearms, by a wide margin, to the U.S. than any other country. See page 20 in the report below.
http://www.atf.gov/publications/firearms/121611-firearms-commerce-2011.pdf
At about 740,000, firearm imports from Brazil actually make up over a quarter of the TOTAL firearms imported to the U.S. in 2010. There are at least 3 companies represented in that figure.
Austria's next in line with 15% of the total and Italy and Germany are 3rd and 4th with about 10% and 9% respectively. Croatia comes in 5th with about 8%.
What's interesting about Croatia is that ALL of the imported firearms from there are handguns. I'm betting that they're all headed to Springfield Armory for rebadging as XD pistols.
Austria's firearms imports are nearly all pistols as well.
At any rate, those five countries, together, make up almost 70% of all firearms imported to the U.S.
For comparison, firearms imports in 2010 were about 3 million. Domestic manufacturers make between 3 million and 5.5 million firearms annually and only about 200,000 to 400,000 firearms are exported from the U.S. annually.
In the U.S., the largest firearms manufacturer (based on 2009 figures) is Sturm Ruger, by a runaway. At about 924,000 firearms in 2009, they made about a sixth (17%) of the domestically manufactured firearms in the U.S. that year.
S&W and Remington were virtually tied for second at about 612,000 each with Maverick in fourth at about 425,000 and Marlin and SIG not far apart in fifth (269,000) and sixth (256,000) respectively.
http://www.atf.gov/statistics/download/afmer/2009-firearms-manufacturers-export-report.pdf