PS: For a somewhat less contentious example, video games have been linked with a significant drop in crime. However, because that does not agree with their biases people focus on the direct effects to support their argument.
I don't think an unbiased study has been done that accurately identifies causes looking at gun ownership and crime. My suspicion is that it would be possible to do, as I described above.
This is a pretty minor effect, unless you are including suicides. In 2007, there were only 613 accidental firearm deaths. For comparison, there were 1039 "transport, other" deaths (I believe this means plane/boat crashes).
All homicides Number of deaths: 18,361
Firearm homicides Number of deaths: 12,632
vs: Motor vehicle deaths 33,808
Death rate extrapolations for USA for Smoking: 440,000 per year (probably exagurated) http://www.wrongdiagnosis.com/s/smoking/deaths.htm
http://www.amazon.com/More-Guns-Less-Crime-Understanding/dp/...
(I haven't read it myself, so I'm not endorsing it. It's not exactly uncontroversial.)
However, the scientific criticisms of his methodology hardly look damning (according to wikipedia). It doesn't look perfect, but it doesn't look terrible either.
The criticisms wikipedia lists: he used cocaine prices rather than cocaine consumption as a variable, if you remove smaller counties +florida the drop in murders becomes statistically insignificant (drops in assault and robbery are still significant), and a couple of critics believe the model has bias and coding errors. As I said, I haven't read it in detail, so I won't comment on how significant these are.