NICS

In the seven and a half years since Obama became President, there have been 130 million background checks performed through the NICS system. That is compared to the 78 million that were performed in the 11 years prior to that. (source here)

Now the number of NICS checks doesn't always mean that the same number of guns were sold. Each check may be for more than one gun, as the check is done for each transaction, not each firearm. With that in mind, we use this as an approximation for the number of guns sold. Even so, the fact that there have been over 200 million guns sold in the past 18 years makes me seriously doubt that there are only 300 million firearms in the US.

If I were to guess, I would say that there are more like 400 or 500 million firearms in this country. Tell me again how you are going to confiscate all firearms, when you can't even tell me how many you want to confiscate, especially when there are hundreds of millions of them.