OK, one study I found so far:
http://pediatrics.aappublications.or...full/104/3/530
The mean number of guns per gun-owning household was 2.7
No idea how accurate it is.
But THIS is really interesting. this is a number quoted by a group that WANTS everyone to own guns (and protect their right to them).
http://www.saf.org/LawReviews/Kleck3.htm
...there are an average of about 5 guns per gun-owning household in the US (Kleck 1997, p. 69)
Given that number, it definitely changes the results in Freakonomics (and I'm sure there are other variables as well that they missed).