The latest population estimate is 327 million Americans.
According to a recent CBS poll 36% of American households own a gun.
36% of 327 million = 118 million.
Given that it's a CBS poll and gun-owning "households" often have more than one gun owner in residence I think it's fair and conservative to round up to 120 million.
Just thought some fresh data might warm up the conversation. ;-)
The more the merrier. :-)