So the study found a correlation between suicide by firearm and violent crime, not firearm prevalence and violent crime. They try to justify using suicide by firearm as a stand in for gun ownership, but if they don't have good stats on gun ownership to use for this study, then how could the previous studies have been able to make the connection between suicide by firearm and prevalence since they would need the same data.

>Firearm prevalence in the United States is difficult to determine because there is no database that collects information on firearm ownership and prevalence. Thus, analyses that study firearm prevalence have had to develop proxies for firearm ownership. As a proxy for firearm ownership, the current analysis used the percentage of suicides by a firearm from 2000 to 2010.

Little tricks like this are why people lose faith in science.