At the height of U.S. involvement in Iraq, the U.S. armed forces were using 1.8 billion rounds of small-arms ammunition every year. In 2005, it got so bad that American ammunition makers couldn't keep up with demand. The U.S. had to import more supplies from Israel.
John Pike, director of GlobalSecurity.org (a DC-based public policy organization) came up with the 250,000 number based on figures from the U.S. Government Accountability Office. It's only a rough, conservative estimate, since the U.S. military doesn't officially keep a body count. According to Pike, that number may actually be as high as 300,000 bullets per insurgent killed.