Here is a great article that I find the fact to be incredible and makes the number that we keep posting to be very low.
In this article it shows how many guns where sold from 2005 to 2014 which 2014 is still not over since the largest sales of guns is Christmas gifts. So only the last ten years the number are 55.08 million guns. So the 300 million is more like 400 million or more, We just do not know!
Here is the link, go down to the stats.