They're all self reported, to start with.
Remember back in October when they were saying the same thing about the electoral polls? "They can't be right because they're only talking to some people"? Stuff like that.
The metrics of social measurement and demographics have gotten really really good. The companies that do this stuff have refined their techniques to an amazing extent.
Even NPR, which typically
NPR is a source for conventional wisdom, nothing more. It's no less corporate than the other networks, just more genteel. In parts of the country, NPR is as pro-gun as Fox or the Fox Street Journal.
But if, as you say, the statistics are inconclusive, then at best we're probably looking at pretty flat gun ownership. I believe about 90% of the gun news we hear is marketing for the arms manufacturers who are looking at a somewhat saturated market. Thus the notion that someone who buys a gun for protection really needs several guns (as a gun owner myself, I see the magazines and read the articles/PR-stuff) That's why they're starting to market their products to 5 year-olds in the form of "The Cricket - Your First Gun" which has already cost a few toddlers their lives.
A very ugly business that makes the US the laughing-stock of the world.