ArchPundit has a thoughtful analysis of the latest from Reynolds and Lott.
Shorter dsquared: If you use some data to construct a model, then to test it properly you need new data. Lott's approach is a little different. The model that was given greatest prominence in the original Lott and Mustard paper (the results of this model where given in the abstract and whenever Lott summarized his paper) showed that there was a 3.5% decline in the violent crime rate associated with the carry laws. In the second edition of More Guns, Less Crime Lott has a chapter where he gives the results he obtained by adding four more years of data. Except that he uses a different model. Nowhere does he present the results you get with the original model. Why not? A clue to the mystery appears in Ayres and Donohue's paper. Table 10 shows that with five extra years of data, Lott's original model indicates that the change in the violent crime rate associated with the carry laws was 0.0%.