Two characteristics normally identify a FUDD.
#1: Very limited range of shooting interests. A FUDD could have just 1 gun, or more than 153 guns, but all those guns fall within a very limited range of interests. The interest that defines a FUDD is "sporting purposes". All their 12 ga wood and blued steel shotguns, bolt action rifles, 44 magnum revolvers, and 9mm pistols only have one purpose: sport. These guns are either for hunting, or they are for marksmanship and trap competitions. Nothing they have is supposed to take the role of a dedicated defense gun, or a daily carry gun. They wouldn't be caught dead with anything black polymer, tactical, or concealable. If you ask them why they own guns, the typical answer is for hunting or for sport. If you ask them about personal defense, expect either half-assed answer about a shotgun loaded with birdshot, or just calling the Police.
Here's a fictional FUDD:
Name: Ed Mulder
Employment: Auto Mechanic
Guns: 3; 1 Mauser 98 sporter, 1 Remmington 870 shotgun, and 1 Ruger 10/22.
Why he owns guns: He likes the sport.
Reaction when offered an AR-15: Grimmace.
Stance on Second Amendment: Giving citizens the right to have guns, but allowing for certain types of dangerous guns, like handguns and semiautomatic rifles, to be regulated. All he cares is that he still gets to hunt and do shooting sports.