A majority of the residents of Civil War era New York all carry small single-shot boot pistols for protection
Percussion Pocket Pistol
When Happy Jack Mulraney (John C. Reilly) is chasing Amsterdam (Leonardo DiCaprio) through the church, he draws a Percussion Pocket Pistol
Based on the barrel look it looks to be a Queen Anne pistol, or if indeed being a pocket pistol, the Liegi derringer
Colt Model 1849 Pocket Model
During the Theatre Scene an Assassin (Bronco Mcloughlin) attempts to kill Bill "The Butcher" Cutting (Daniel Day-Lewis) with a Colt 1849 Pocket Model Revolver
Double Barrel Shotgun
Seen being held by a Ship Captain and William Tweed (Jim Broadbent).
Elgin Cutlass Pistol
Amsterdam (Leonardo DiCaprio) attempts to assassinate Bill "The Butcher" Cutting (Daniel Day-Lewis) by throwing a knife and then pulling an Elgin Cutlass Pistol, only to be impaled by a knife thrown by Cutting
Colt 1851 Navy Revolver
During the Draft Riots Scene, a mob breaks into Mr. Schermerhorn's (David Hemmings) house, and he in turn uses two Colt 1851 Navy Revolvers akimbo style
Joslyn Rifle
During the scene where William Tweed (Jim Broadbent) is conversing with Dead Rabbit supporters, Jenny Everdeane (Cameron Diaz) keeps a Sharps Rifle aimed on him
Springfield Model 1842 Musket
An Irish imigrant is seen being issued a Springfield Model 1842 Musket. This is historically correct, as the Irish Brigade were issued the 1842 Musket until 1864, as it was preferred for it's ability to use the "buck and ball" load.
Some US Army troops are seen carrying Enfield Pattern 1853 rifles, which were commonly issued to Union regiments.
Unknown Colt Cap & Ball Revolver
During the scene where William Tweed (Jim Broadbent) is conversing with Dead Rabbits, Amsterdam (Leonardo DiCaprio) appears to have a Colt Cap & Ball Revolver tucked in his waistband, the same gun is seen, during the debate scene.
There is a good possibility that this is the 1849 Pocket Model, that he took off the assassin who tried to kill Bill earlier.
Unknown Firearms
There are numerous firearms in the movie that are hard to identify
Ship Cannons
During the Draft Riot, just before the fight between the Dead Rabbits and the Bower Boys, a ship of the coast fires its cannons off.