There are two common types of handguns: revolvers and semi-automatic pistols. Revolvers have a number of firing chambers or "charge holes" in a revolving cylinder; each chamber in the cylinder is loaded with a single cartridge or charge. Semi-automatic pistols have a single fixed firing-chamber machined into the rear of the barrel, and a magazine so they can be used to fire more than one round. Each press of the trigger fires a cartridge, using the energy of the cartridge to activate a mechanism so that the next cartridge may be fired immediately. This is opposed to "double-action" revolvers, which accomplish the same end using a mechanical action linked to the trigger pull.