A revolver uses a cylinder with holes drilled in it for the ammunition to fit in. Older models had to be packed with a led ball and black powder. Modern revolvers use bullet cartridge. When the trigger is pulled an arm pushes against catches in the cylinder rotating it around to the next cartridge. In modern revolvers the hammer is pulled back by pulling the trigger as well. Older models require the hammer to be manually pulled into position. The trigger activates the hammer which sets off the primer charge in the bullet cartridge and fires the revolver.