Gallup is probably closest to accurate. The lower numbers are all from polling by gun control organizations - and are used to push the agenda/meme that gun culture and gun ownership are a dying thing of the past.
I can't imagine news organizations using numbers from an NRA sponsored poll. The poll numbers provided by the gun control advocates are ludicrous as even in the face of a huge increases in guns sales the last 4 years with many first time buyers they continue to claim a steady decrease in gun ownership. But none the less the media keep using their numbers as gospel.
The truth is that Gallup numbers are probably low - as has been stated in many areas of the country gun ownership has been stigmatized so people will sometimes deny owning guns when they do. Also there is the feeling amongst a portion of the gun community, as in those that buy guns only in private sales, that do not talk freely about owning guns to anyone that they do not know and trust. Those groups of individuals as well as those with an old family heirloom that they may forget about are all likely to say no to a survey and thus result in significant under counting.
Given roughly 30 plus some percent of individuals owning guns and the report of 40 plus some of households owning guns; a reasonable estimate of the number of gun owners in this country is give or take a few million about 100 million gun owners.
The number of guns owned is also problematic as it is also highly susceptible to under counting. Think about it - some gun owners on here couldn't tell you off the top of their head how many guns they own, some don't tell their spouse or family or friends how many guns they own. Some of the survey reports answers are based on the reports of members of the household other than the actual gun owner. How many times if asked "how many guns do you have?" have you yourself said or heard someone else say in response - "a couple" - "a few" - "oh, one or two." Yes, some gun owners will talk expansively about the guns they own, but just as many if not more, especially to a stranger on the phone will minimize and under report.
The estimate of 270 million is from the small arms survey last done in 2007, so there has been 5 to 6 years of Gun sales between then and now. There have been well over 70 million NICS checks from 2007 through 2012.
http://www.businessinsider.com/number-of-guns-sold-in-us-each-year-2012-12
Even conservatively estimating that half those checks were for used firearms and that they only represent one gun sold for each transaction (some are multiple sales) that would be another 35 million guns.
So it is safe to say there are more than 300 million guns and probably multiples of millions more in private hands today.
The above combined with over 8 million licensed CCW holders, not counting the states that don't require a license and those that allow open carry, along with reports of increased female participation in hunting, shooting sports, CCW applications, and reports of increased CCW applications across the board all argue that the number of Americans who own and use guns is not decreasing like gun control advocates claim - but is actually increasing.
Anecdotally, I am aware of friends and acquaintances of a generally liberal persuasion who own guns but would not acknowledge such to friends or co-workers as it is not PC in their work and peer group. I only know because I am a "gun guy" and they have talked to me on the QT about buying a gun or getting another gun, etc... Heck just think of all the news stories over the years of prominent gun control advocates involved in problems where it becomes public knowledge that despite their advocacy they personally own guns.
Just another example of the media reporting what they want to be true instead of actually investigating the facts to get to the truth. Widespread gun ownership would hinder the agenda of gun control advocates to further stigmatize gun ownership as an increasingly aberrant and irrelevant cultural artifact of the past that needs to disappear so "modern civilized society" may move on.
This is the same reason that gun control or gun ban advocates studiously ignore or do all in their power to minimize defensive gun use.
