Revolvers shoot bullets. Sneak + right click to reload. Although all four types of bullets that a gun can use stack to 64, the gun can only hold 8, meaning that it must be reloaded regularly. It can shoot: Bullets, Golden Bullets, Magic Bullets, and Fire Bullets, but not cannon bullets. Different bullets have different effects on the target.
You must have bullets in your inventory for the Revolver to work. When you have the Revolver selected, Shift + Right-Click to open up the Revolver GUI. In the GUI you will find a slot to drop bullets into, to load the Revolver. Once your Revolver is loaded you may fire it by right-clicking.