There are single action triggers and double-action triggers on revolvers. On a single-action revolver you pull back the hammer to advance the cylinder to the next round. There are no single-action semi-automatics, that would be a non-sequitor. A revolver is a very reliable mechanical way to fire a gun and there are ways to speed load them. In a semi-automatic a build up of exhaust gas, if not cleaned, can lead to jams. Also a re-loaded shell with 'not enough gunpowder' can lead to a jam.