A replacement slide assembly for a pistol, the slide assembly having a barrel assembly with a barrel and a firing chamber. The barrel assembly also has a guide member which is a protrusion that extends horizontally outward from the barrel assembly. The replacement slide assembly also includes a housing with an interior wall that defines a groove that receives the guide member. The guide member and the groove in the housing are located forward of a firing pin assembly also mounted within the housing. The slide assembly or any firearm also includes an adjustable sight assembly that permits interchangeability between different types of sights without requiring that the firearm be re-sighted.