It is a "turret pistol." It holds seven cartridges. This gun is cocked by squeezing in the lever sticking out on the right, and is fired by pressing a small trigger lever just under the barrel. What appears to be an abrasive wheel in the center is a knurled inset that the user rests their thumb against to steady the gun when cocking and firing.

You would place the gun in the palm of the right hand, with the curved lever against the heel of the hand. Then wrap your fingers around it with the barrel protruding between the index and middle finger.