I think for every known gun owner, there is at least one closet gun owner. It's not PC to be known as a gun owner, kids won't be allowed to come over to play with the kids of known gun owners, etc.
Think of it like this, we know there has to be at least 10,000,000 gun owners, but there are only 4,000,000 NRA members, why? Same equation. I figure that 50% that vote the conservative ticket are gun owners, and 20% of the liberals are also.
50% of 56,000,000 = 28,000,000 and 20% of 63,000,000 = 13,000,000 so by this very crude and total imaginary calculation we have 41,000,000 gun owners in the USA of which only 10% belong to the NRA.
These numbers make sense when the figure for weapons in the hands of US citizens stands at approximately 750,000,000 Actually, they make little sense and it means that if the 750,000,000 number is correct then it would make more sense that at least double the amount of Americans are gun owners.