Strike one for the good guys.

[daishi5]

Originally posted by: Red Dawn
So there's no documented source, got it.

Rather than make you search on your own, I did grab a section of the suggested reading material that I believe gives a small insight into the debate.

How many times each year do civilians use firearms defensively? The answers provided to this seemingly simple question have been confusing. Consider the findings from two of the most widely cited studies in the field: McDowall et al. (1998), using the data from 1992 and 1994 waves of the National Crime Victimization Survey (NCVS), found roughly 116,000 defensive gun uses per year, and Kleck and Gertz (1995), using data from the 1993 National Self-Defense Survey (NSDS), found around 2.5 million defensive gun uses each year.

Many other surveys provide information on the prevalence of defensive gun use. Using the original National Crime Survey, McDowall and Wiersema (1994) estimate 64,615 annual incidents from 1987 to 1990. At least 19 other surveys have resulted in estimated numbers of defensive gun uses that are similar (i.e., statistically indistinguishable) to the results founds by Kleck and Gertz. No other surveys have found numbers consistent with the NCVS (other gun use surveys are reviewed in Kleck and Gertz, 1995, and Kleck, 2001a).

To characterize the wide gap in the estimated prevalence rate, it is sufficient to consider the estimates derived from the NSDS and recent waves of the NCVS. These two estimates differ by a factor of nearly 22. While strikingly large, the difference in the estimated prevalence rate should, in fact, come as no surprise. As revealed in Table 5-1, the two surveys are markedly different, covering different populations, interviewing respondents by different methods, using different recall periods, and asking different questions.

The data is very strange, apparently we have estimates of:
67,000 per year
116,000 per year
And then we have about 20 studies that all find somewhere around:
2,500,000

Unfortunately, I am not familiar with the other 19 studies, and the preview from their website does not make it clear how trustworthy those studies are. The fact that 20 of them were almost the same results however does lend them some weight. From another section they discuss why the numbers appear to be so different

The discrepancies in the prevalence estimates of defensive gun use can and should be better understood. Remarkably little scientific research has been conducted to evaluate the validity of DGU estimates, yet the possible explanations are relatively easy to categorize and study. The two surveys are either (1) measuring something different or (2) affected by response problems in different ways, or (3) both. Statistical variability, usually reflected by the standard error or confidence interval of the parameter, also plays some role but cannot explain these order of magnitude differences.

Coverage
Perhaps the most obvious explanation for the wide variation in the range of DGU estimates is that the surveys measure different variables. In the NSDS, for example, all respondents are asked the gun use questions. In contrast, the NCVS inquires only about use among persons who claim to be victims of rape, assault, burglary, personal and household larceny, and car theft. The NCVS excludes preemptive uses of firearms, uses that occur in crimes not screened for in the survey (e.g., commercial robbery, trespassing, and arson), and uses for crimes not revealed by respondents.1

McDowall et al. (2000) found some evidence that these differences in coverage play an important role. In an experimental survey that overrepresents firearms owners, 3,006 respondents were asked both sets of questions about defensive gun use, with random variation in which questions came first in the interview. By holding the survey sampling procedures constant (e.g., consistent confidentiality concerns and recall periods), the authors focus on the effects of questionnaire content. Overall, in this experiment, the NCVS survey items yielded three times fewer reports of defensive gun use than questionnaires that ask all respondents about defensive uses.

The McDowall et al. (2000) crossover experiment is informative and is exactly the type of methodological research that will begin to explain the sharp divergence in gun use estimates and how best to measure defensive gun use. There remains, however, much work to be done. The sample used

So, it appears that one survey, (the one with an estimate of 116,000) will underestimate the defensive use of guns because many people are never asked, so that gives us a reasonable lower bound, and the other surveys, (2.5 million) overestimate the use of guns, because people include more than actual defensive gun uses within the time frame they want. That gives us an upper bound. So, 1 study is too low with 116,000, 20 studies are too high with 2.5. We can assume the number is probably between those two, and with so many of the studies being on the very high side, I would guess that the higher number is much closer to the truth, but I don't know of any way of confirming that with the current data that I am aware of.

Source