So the CDC arraigned for a group to to conduct one.
Among the findings noted in this initial study:
Almost all national survey estimates indicate that defensive gun uses by victims are at least as common as offensive uses by criminals, with estimates of annual uses ranging from about 500,000 to more than 3 million per year, in the context of about 300,000 violent crimes involving firearms in 2008.
The New American offers this recap of the study.
For those policy nerds who want the whole unedited caboodle - knock yourselves out here.