I suppose someone could construe facts and data as opinions if they do not like the facts and data.
I guess this is a rant, but I am going to take a stab in the dark and say that this is not my opinion regarding where the Guardian got their data and that the Harvard/Northeastern survey was a survey:
From the Guardian: The unpublished Harvard/Northeastern survey result summary, obtained exclusively by the Guardian and the Trace, estimates that America’s gun stock has increased by 70m guns since 1994. At the same time, the percentage of Americans who own guns decreased slightly from 25% to 22%.
Source: "The Stock and Flow of US Firearms: Results from the 2015 National Firearm Survey"
From their exclusive partner, the Tace: It was conducted using the GfK Knowledge Panel, a nationally representative online panel used by researchers and marketing companies to poll Americans on a variety of issues. It is undergoing peer-review, and a summary will be published by the Russell Sage Foundation next year.