Arms Dealer

The Arms Dealer is an NPC vendor that will spawn once the following criteria have been met:
 * There is an empty house.
 * You have at least one bullet firing Gun or Bullet in your inventory (not just stored in a Chest).

The easiest way to acquire a qualifying item is to smash either Shadow Orbs or Crimson Hearts in the Corruption or the Crimson. They may drop a Musket or an Undertaker, respectively. Guns stored in Chests will not fulfill this requirement, nor will magic guns such as the Space Gun or the Laser Rifle.

If the Arms Dealer is killed, he will only respawn if the above conditions are met again. This means you must place a Gun or Bullet in your inventory.

When threatened by enemies, the Arms Dealer will use a Flintlock Pistol to defend himself. Once Hardmode begins, he upgrades to a Minishark for self-defense.

Names
The arms dealer may have any of the following names:

Quotes

 * "Keep your hands off my gun, buddy!"
 * "Hey, this ain't a movie, pal. Ammo costs extra."
 * "I see you're eyeballing the Minishark. You really don't want to know how it was made."
 * "Don't bother with [Name of Demolitionist], I've got all you need right here." (Only if Demolitionist is present.)
 * "What's [Name of Demolitionist]'s problem? Does he even realize we sell completely different stuff?" (Only if Demolitionist is present.)
 * "Make it quick! I've got a date with [Name of Nurse] in an hour." (Only if Nurse is present.)
 * "I want what [Name of Nurse] is sellin'. What do you mean, she doesn't sell anything?" (Only if Nurse is present. Also refers to the fact that the Nurse doesn't sell anything.)
 * "[Name of Dryad] is a looker. Too bad she's such a prude." (Only if Dryad is present.)

Only during night:
 * "I heard there is a doll that looks very similar to [Name of Guide] somewhere in the underworld. I'd like to put a few rounds in it." (This will be the only message available if Skeletron is defeated, but Wall of Flesh is not.) (This refers to the Guide Voodoo Doll)

Only during a Blood Moon:
 * "I love nights like tonight, there is never a shortage of things to kill."
 * "Man, it's a good night not to talk to anybody, don't you think, [Name of Player]?" (This is a reference to the NPCs becoming incredibly paranoid, sinister or rude during Blood Moons.)

Trivia

 * The Arms Dealer is very arrogant compared to the other NPCs and seems to have an observant personality, often noticing you (apparently) intend to either buy the Minishark or steal one of his weapons. This, when coupled with the way he speaks, may support that he is based off the traditional gangster.
 * He seems to have a rivalry with the Demolitionist, as he says his products are better than "fireworks". He also has a crush on the Nurse according to his quotes.
 * The name Brimst may be a reference to brimstone (sulfur), one of the ingredients of gunpowder.
 * The Arms Dealer's sprite is very similar to that of the Demolitionist's alpha version sprite.
 * Unlike several other NPCs, who sell their own clothes on Halloween, the Arms Dealer sells the Nurse's clothes, further cementing his unhealthy obsession with her. On a technical level, this is likely because the Nurse does not have a shop interface through which to sell her own clothes.
 * The Arms Dealer may now spawn after defeating 50 Hellbats even if you do not have a gun in your inventory