Gun and ammo sales
In the last three months of 2008, that's right, three months, Americans bought enough guns to outfit the Chinese and Indian armies, combined. Actually, that's just reflective of the NICS checks, which don't cover sales by private parties, and buys by CCW permit holders. There were also 1.5 BILLION rounds of ammo sold, which explains why I'm having trouble finding .45 ACP lately.
Hat tip to reader Joe Olson, who also points out that a large share of those firearm sales were of "assault rifles," which means it'll be hard to argue that they don't meet the Heller test of arms "in common use."
UPDATE: Here's a link to some pdfs breaking down background checks by year, State, and State rate per capita.