Guns

Guns are ranged weapons that fire bullets or comparable items as ammunition.

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

Several other items are referred to as "guns" in-game (e.g. Piranha Gun, Space Gun, or Water Gun), but none of these fire bullets or gain the above bonuses related to guns, nor will they cause the Arms Dealer to spawn.

The best modifier for guns is Unreal, with the exception of guns without knockback

Types
There are currently different guns available in Terraria,  of which are Hardmode-exclusive.