Abstract: A caliber-reducing adaptor kit for a revolver having an original revolver barrel, and a cylinder with a chamber having a powder-receiving compartment at one end. A substitute barrel, having a reduced caliber bore, is received in and is detachably mounted to the original revolver barrel. A cylinder insert is removably received in the cylinder chamber between the substitute barrel and the powder-receiving compartment, the insert having a caliber bore substantially the caliber of the substitute barrel. The cylinder insert includes a plunger having a head at one end engageable with an end portion of the substitute barrel, and having a shank portion, the bore being provided in the plunger. A sleeve is slidably mounted on the shank portion of the plunger and is engageable with the cylinder adjacent the powder-receiving compartment.