The weapons brought along by Hanover (Wes Studi) and his team of pirates to loot the cruise ship Argonautica are described in the movie as "Chinese M1-L1 triple-pulse assault rifles" which have "thousand-round capacities", rotating barrels, and are "air-cooled & water-tight".
No such weapon exists in reality. The guns used to build the prop pulse rifles ("practical weapons") are 9mm Calico M955A submachine guns with 100-round magazines. Among the many modifications that appear to have been added by the film's armorers are "thumbhole" stocks, large flashlights, and claw mounts fitted with some type of red-dot reflex sight. The five rotating barrels on each gun do not actually fire; they are fitted around the real barrel of each Calico and seem to be driven by a small electric motor connected to the trigger, so that when the guns are fired, the barrels will spin. The enormous muzzle flash of the Calicos (probably generated with full-load blanks) usually obscures this special effects trick, but in several scenes of the movie, it is possible to see that the muzzle flash is coming from between the rotating barrels, not through them. Several screenshots seen below illustrate how this was done.
Taurus PT92/PT99
A pair of Taurus PT92-type 9mm pistols are used frequently throughout the movie.
Hanover (Wes Studi) carries a black PT99, a variant of the PT92 which has adjustable sights, as his sidearm. He's frequently seen using it to threaten people who disagree with him (most notably Mulligan, near the end of the film). For some reason, he is always seen reloading the gun after encounters with the monster's tentacles, even though he never fires it once on screen. In fact, after the group's first encounter with the beast, he is seen reloading the gun THREE times in a row.
Vivo (Djimon Hounsou) carries a stainless PT92 AFS as his sidearm, which winds up changing hands numerous times throughout the movie. When Vivo is killed, this gun is taken by Hanover and is later stolen by Trillian (Famke Janssen), who uses it to dispatch the monster's tentacles in order to save Finnegan (Treat Williams). Later, Vivo's PT92 ends up in the hands of Joey (Kevin J. O'Connor), who gives it back to Hanover when he is in the process of being painfully devoured by the monster (which kills its victims by slowly sucking fluids out of their bodies). Hanover shows his gratitude by firing a round at Joey (who runs off), but when he tries to commit suicide with the gun, the hammer falls on an empty chamber.
An interesting continuity error is that in some scenes, the stainless PT92 becomes a Beretta 92FS Inox, as explained below.
Beretta 92FS Inox
In some scenes of the film, the stainless Taurus PT92 that is wielded by many of the film's characters becomes a Beretta 92FS Inox. For instance, when Finnegan takes the pistol from Trillian before diving into the submerged tunnel, it quite clearly has the slide-mounted safety, which would make it a Beretta 92FS rather than a Taurus PT92, which has the safety mounted on the frame.
Walther PPK (?)
When Finnegan tries to make the pirates stop beating up Joey and hits one of them in the face, two of them, including Mason (Clifton Powell) and T-Ray (Trevor Goddard) draw their sidearms on him. In this scene, T-Ray is shown holding what appears to be an early-model Walther PPK, but it's hard to be sure because the gun is never seen very clearly.
Beretta 92FS
Mason (Clifton Powell) wields a standard blued Beretta 92FS with after-market wood grips in the scene where Finnegan (Treat Williams) confronts the pirates. It is not seen again after this encounter.
Ithaca Model 37 (sawed-off)
Finnegan (Treat Williams) carries a short barreled Ithaca Model 37. Finnegan uses this shotgun most notably in the climax after encountering the massive, ugly head of the monster in the atrium of the Argonautica. When the monster brings one of its huge blue eyes down to look at him, he tells it, "What're you lookin' at?" before drawing his Ithaca and giving it a blast of 12-gauge buckshot in the eye.
Browning M2 Aircraft
The stern of Finnegan's Air-Sea Rescue boat, the Seapan, contains a mounted machine gun of some type. Since the Air-Sea boat is a WWII-era vessel, it's likely that it's a Browning M2, but this is uncertain. The gun is never fired, and can only be seen (barely) in a few shots of the boat.