I think its not just the amount of guns but the majority of the problem is education, culture, and the differences in society between the USA and...say Switzerland.
Switzerland are noted as 3rd on the graphic I posted as guns-per-head of population...but they also don't have an army and rely on a 'home militia' type system, so people keep their guns at home as part of the actual "defence force" if you can call it that.
I had a look at some stats and Switzerland has about 1/40th the population of the USA but 1/258th the amount of people killed from guns.
I know Switzerland, even though high on the list still has a smaller guns-per-head compared to the USA, but going by these stats you could say that Switzerland could up their guns to more than the Americans for guns-per-head and they would still have far less gun killings than the USA.
Surely this means that accessibility to guns isn't the main cause but other factors contribute more to the use of guns to kill people.
Its why I am not that fussed about "tighter gun controls" as people will still get access to gun but I think a lot of it comes down to the surrounds/environment, the culture, the people and their attitudes towards gun crime and gun killings.