About a million. You get somewhat different figures from different sources - ranging from around 730,000 as you mention from a US study done by the Library of Congress, up to 1.2 million by a 2011 study done here in Australia by Bauer and English, so 'about a million' is a decent midrange figure.
The number of legal firearms in 2012 was 2,750,000.
Close enough - 3 million is a more common estimate, but that's not that different.
About 172,000 hand guns are owned by the people.
That's about right - these get counted properly, because of the category licences and permits, so that's a fairly solid number.
Since you are a professional military historian living in Australia, I thought that you would post the precise figures and sources, if you disagreed with the numbers that I found, and preferably for 2013 instead of 2012, as I posted.
As someone who claims to be a professional military historian, you sure seem to dislike to post facts and sources, until you do I will have to stick with the figures that I have seen so far.