The following guns were seen in the movie Constantine:
John Constantine's Holy Shotgun
The Holy Shotgun is seen as a firearm constructed out of Holy Relics, firing gold rifled slugs encased in engraved 12 gauge sized shells. It also has the "Dragon's Breath" flamethrower (see below) mounted beneath the barrel. Though based heavily on the Armsel Striker 12/SWD StreetSweeper shotgun, it is not built on either one. The receiver is radically different in size and shape (quite a bit smaller and smoother on Constantine's gun). Also the ornately engraved drum magazines are shown as quick detachable and reloads (much like the drum magazine of a PPsH-41) but it 'spins' like the cylinder of a Single Action Army Revolver. None of the real world weapons have this capability, so it is a completely built prop weapon, using SOME real world parts, but for the most part is fictional.
An interesting note: Whereas Angela Dodson's (Rachel Weisz) S&W 3913 pistol fires a huge amount of shots without reloading, Constantine's shotgun has a ammo deficit. Despite each drum holding twelve rounds, Constantine only fires eight shots before he has to reload, and after reloading the second drum, he runs out of ammo in five shots. Either an editing mistake, or he has really bad primers....
Dragon's Breath Flamethrower
An ornate tube that shoots fire, given to Constantine by Beeman (Max Baker).
Mossberg 500 "Cruiser"
Smith & Wesson 3913
Angela Dodson (Rachel Weisz) uses a Smith & Wesson 3913 in several scenes, shown to be very ineffective against demons. During the fight in the pool, Angela's Smith & Wesson 3913 fires 33 shots without reloading, which is pretty good for a pistol that holds 8 rounds in the magazine.