I think the point it is making is that there are a lot of people with a lot of guns in the US.
I can’t find the 2007 survey they refer to but the 2002 survey is here:
That seems to be focussed on small arms numbers (and prices, well supply generally) because that is what governments were worrying about.
They make some statements about for example being able to gauge the volatility of the political situation or the desire for insurgency, etc. by the price of fire-arms in a region. If Columbian rebels are paying a fortune for AK-47s, they evidently want them quite badly. If they could buy more, for less - presumably they would use them more, etc. Arms prices in Afghanistan tell you how likely an upsurge in violence is and so on.