Abstract: A breech plate and cylinder for the conversion of a percussion pistol to a cartridge pistol which requires no modification of nor permanent attachment to the frame of the pistol. The breech plate is non-rotatably mounted within the cylinder receiving area and includes a firing pin in proper relation to the cartridges loaded into the cylinder and the cylinder and breech plate are held within this receiving area through the normally provided cylinder pin. The breech plate is stationary and secured against rotation with respect to the cylinder and cylinder pin by, selectively providing, pins or locating flats to eliminate any frame modification or attachment to the frame. The cylinder is rotatable about the cylinder pin to sequentially position cartridges in firing location with regard to the breech plate carried firing pin and barrel of the pistol.