The Reason for Our Woeful Lack of Gun Statistics

How often are guns used for self-defense? Advocates claim as often as 2.5 million times a year. That's an incredible figure, for reasons explained here and here. On the other hand, the number of defensive gun uses must surely be much higher than zero. It would be good to know how much higher. Perhaps some competent agency might do some better-quality research? The Centers for Disease Control would seem one logical candidate, they do many of the nation's best public-health studies.