Here in New Jersey, it's probably either a pump-action shotgun, a 4" .38 caliber revolver, or a 9mm pistol - most likely a Glock 19, Beretta 92FS, or Sig P226.
Why am I so confident? The hunters have the shotguns (you can't hunt with a rifle in NJ). The older residents have the old .38 caliber revolvers. The younger people have the 9mm Glock 19's and Beretta 92's. So why am I being so specific?
NJ has a magazine capacity limit of 15 rounds. If you buy a Glock 17 or an M&P9 the gun comes with 10 round magazines. So (at least in NJ), a Glock 19 holds more rounds than a Glock 17. The Glock 19, Beretta 92 and Sig P226 come with 15 round magazines standard.
I would imagine that in states with 10 round limitations, you would see people go with an increase in caliber to compensate for the limited magazines. This leads me to believe that in states with 10 round limitations, you would see more .40's, .45's or .44 wheelguns, but I could be wrong.
I know a couple of people who only have one gun in their home. Those guns are:
Mossberg 590.
Sig P220 38 Super (An odd caliber, but she likes it)
Glock 19
Colt Python
Kimber .45
Remmington 870
Glock 22
Glock 22
Sig P226
Sig P226