The Swiss Graduate Institute of International Studies just released its 2007 Small Arms Survey.
Using a variety of methodologies, it ranks countries by the proportion of the civilian population with guns.
Confirming the centrality of its loopy gun culture, the US is top in terms of both the total number of firearms, and the number of guns per head of population. By a long shot.
In the US, there are 90 guns for every 100 civilians! That's nine for every 10 men, women, children and babies! For a grand total of 270 million guns.
Second, per capita: Yemen (!), with 61 guns per hundred people, then Finland (!!), with 56. Unsurprising, recent conflict results in a deluge of guns: Iraq is fifth (39), Serbia sixth (38).
In the English-speaking world, Canada has 31 guns per hundred people, Australia has a surprisingly high 15, while England has an eminently sensible 6.
A gun culture is a travesty. Unfortunately the world is flooded with it, courtesy of Hollywood and the rest of American culture.
The study doesn't paint a clear link between gun ownership and violence, although I would expect homicides to be well up in countries with high gun ownership. Inter alia, it also says gun ownership increases as the wealth of a country increases [surely with notable exceptions], and it notes that factors in violent cultures include rapid urban growth, poverty, and ineffective policing.
Update 14-Oct-09: Something that may surprise some Americans: New Scientist reports that carrying a gun makes one more than four times more likely to be killed. And when the victim had the chance to defend themselves, the odds of being shot were even higher. As the article mentions, the US has the highest rate of gun homicide in the world.