Lieutenant Helga Katrina Sinclair's weapon of choice is a Mauser C96 Broomhandle which she is seen with it throughout the film waving it around until the climax of the film where she finally uses it to great effect until it runs dry.
Luger P08
Commander Lyle Tiberius Rourke's weapon of choice is a pistol weakly resembling the Luger P08.
Remington 1866 Derringer
The Remington 1866 Derringer is very briefly seen in the hands of Wilhelmina Bertha Packard when the group casts over Milo when they reveal to be mercenaries.
Submachine guns
PPD-34/38
Mole is seen with a submachine gun that weakly resembles the PPD-34/38, fitted with a drum magazine when Rourke reveals to Milo that they are mercenaries, and during the entering of the Kings room. The funny thing is, the PPD-34/38 wasn't available in 1914 and is an anachronism.
The Double Barreled Shotgun, Cookie's weapon of choice, briefly seen when revealing to be a Mercenary and when entering the Kings room shouting "room service".
Machine guns
Lewis Gun
Seen at the climax of the film where the Atlanteans battle Rourke, Helga and the soldiers. Rourke fires a hand-held Lewis gun until it is shot out of his hand by an Atlantean's ship that is piloted by Vinnie.
Vickers machine gun
The Vickers machine gun is seen mounted on the back of the last surviving truck of the mercenaries, where two soldiers are mounting it, never seen being fired. Several mercenaries can be seen firing them during the Atlanteans' assault on Rourke's zeppelin.
Other
Flare pistol
Helga's Flare pistol which she uses on two occasions, the first she fires it up the Volcano and the second firing it at Rourke and the Balloon with famous last words "Nothing personal".
Unknown machine gun
An unknown machine gun is mounted on several trucks and planes (maybe a Browning .30 cal, a few years inaccurate).