A firearm includes a receiver having an opening, at least one cartridge-containing cylinder rotatably disposed within the receiver, and a hammer displaceable between cocked and strike positions. A slide is movably coupled with the receiver so as to be linearly displaceable between a front position and a rear position and displaces the hammer to the cocked position. A rotator is movably coupled with the slide and releasably engages with and rotatably displaces the cylinder when the slide displaces toward the front position and alternatively disengages from the cylinder when the slide displaces toward the rear position. Preferably, each cylinder is rotatably disposed within a chamber of a housing removably disposeable within the receiver cavity and displaceable through the receiver opening. Most preferably, at least one cylinder has chambers for receiving cartridges of a first size and another cylinder has chambers for receiving cartridges of a second, different size.