Guns

Guns are Ranged Weapons that fire Bullets.

In addition to ranged bonuses, guns also benefit from the Shroomite Mask, and are compatible with the Rifle Scope. Possession of any gun (or bullets) fulfills the criteria for the Arms Dealer NPC to appear.

Several other items are referred to as "Guns" in-game, but none of these fire bullets or gain the above bonuses related to guns, nor will they cause the Arms Dealer to spawn.

Other Guns
Despite not firing regular Bullets, some weapons use specialized ammunition which the gun treats as bullet-type damage for specific damage bonuses.

Trivia

 * Most automatic guns are related to an animal (Ex: Gatligator = Gatling Alligator).