Revolvers are the main firearms in Syndicate, six-shot handguns both twins carry alongside derringers and semi-automatic pistols. You use them for Countershots in melee and for picking off armed enemies, and the in-game police carry them too.
The Countershot skill lets you shoot an enemy who is about to fire at you, and Gunslinger I reloads an empty gun mid-combo. Jacob's Gunslinger II turns his Countershots into automatic headshots, which makes him the better gunfighter.
Crawford Starrick owns an M1877 Thunderer. In 1916, Lydia Frye fights with an M1877 Lightning. Syndicate's police also carry revolvers, a choice the developers made for gameplay, since real Victorian officers mostly had truncheons.