In chapter 3 of More Guns, Less Crime Lott presents an analysis based on two exit polls of gun ownership (conducted in 1988 and 1996) that purports to show that a 1% increase in a state’s gun ownership causes a 4.1% decrease in the violent crime rate and a 3.2% decrease in auto theft.

Lott’s two polls indicate that gun ownership increased by a remarkable 50% in just eight years, from 26% to 39%. However, this is contradicted by all other surveys on gun ownership. The best of these are the GSS surveys which actually show a modest decline over that period.

Even Lott found a 50% increase so unlikely that in this Usenet exchange he called it a “strawman”:

Doug Weil:
Two surveys ask questions in very different ways. The organization that collected the data says — the surveys are not comparable. You compare the data anyway, and produce a result so far afield from anything any regularly administered survey has produced (specifically a 50% increase in gun ownership in the general population over an 8 year period all other surveys show no increase in gun ownership) and claim that, well — you must be right because you controlled for differences in the survey questions.
John Lott:
The survey implies at 35% change in gun ownership. Why do I have a feeling that you are trying to exaggerate the poll data so that you can can set up a strawman?

In chapter 9 of the 2nd edition of More Guns, Less Crime Lott defends his use of these exit polls.

Now, in his analysis of safe storage laws in chapter 7 of The Bias Against Guns Lott does not use these exit polls. Instead, despite his suspicion that the GSS surveys are cooked, he uses the GSS surveys to measure how gun ownership changes as a result of a state passing a safe storage law. Using these polls he finds that gun ownership declined by one percentage point per year in the states with the laws and argue that the laws caused increases in crime rates.

Lott does not explain why, after stoutly defending his use of the exit polls to measure changes in gun ownership at the state level he abandoned them for his later paper. One possible explanation is that the exit polls say the opposite thing to the GSS surveys. The exit polls show substantial increases in gun ownership in the states that passed safe storage laws. I computed a regression relating the change in gun ownership as measured by the exit polls to the number of years that a safe storage law had been in place and found that the laws were associated with a 0.06 percentage point per year increase in gun ownership rates. This increase is not statistically significant, but it is the opposite sign to Lott’s result using the GSS surveys.

Since the exit polls show increases in gun ownership while the GSS surveys show decreases, it is plausible that if Lott had conducted his analysis in chapter 3 of More Guns, Less Crime using the GSS surveys he would have found that more guns were associated with more crime.