The improved gun barrel insert is provided in tank-borne guns, field howitzers and the like. It makes possible the use of small caliber and thus less expensive ammunition having ballistic properties of the substituted larger caliber ammunition, while maintaining the opening, closing and igniting mechanisms provided for the larger caliber firing as well as the geometry and weight of the larger caliber cartridges. True combat conditions with a correct observation of shooting regulations can thereby be simulated to a very large extent and a high accuracy in hitting targets from shooting stands or moving bases can be obtained. The provided insert accommodates both a smaller-caliber cartridge and the igniting mechanism therefor. The insert is designed as a slightly tapering rotational symmetrical adapter conformable to the cartridge chamber of the larger caliber weapon. The conical front end of the adapter bears against the mating rear portion of the barrel insert proper. Axially the adapter is held in position by the breech block without any modification.