A mechanism for indexing the movement of a firearm cylinder to load and unload cartridges from a plurality of cartridge-receiving chambers in the cylinder. The firearm may generally comprise a cylinder, a supporting structure rotatably carrying the cylinder and a loading gate cutout. The indexing mechanism may comprise a cylinder indexing member, which in one embodiment may be a stop pin. The cylinder is configured to be stoppingly engaged by the indexing member when the cylinder is rotated in a first direction to align a cartridge-receiving chamber with the loading gate cutout in the frame, thereby creating an indexed position of the cylinder. Preferably, a plurality of indexed positions are created to allow each chamber to be aligned with the loading gate cutout for loading and unloading cartridges. In one embodiment, the cylinder is further freely rotatable in a second direction opposite the first direction without stopping engagement between the cylinder and indexing member. The mechanism may further include a pawl which is moveable into and out of engagement with the cylinder for advancing the cylinder as the firearm is discharged.