Paul Waldman charts it:
The explanations for this drop vary; a declining interest in hunting and the steady exodus from rural areas to suburbs and cities almost certainly play a role. Whatever the combination of causes, there have been steady declines in gun ownership among all age groups. Of particular note is the decline among young adults. In the GSS studies in the 1970s, around 45 percent of respondents under 30 years of age reported that their household owned a gun; in the most recent surveys that number has fallen below 20 percent, a decline of more than half.
Kevin Drum asks:
I'm not sure what's going on. Gun sales to individuals seem like they've increased a fair amount over the past decade, but the number of households reporting gun ownership has decreased a bit. Does this mean that fewer households own guns, but the ones that do own guns have more and more of them?