 The Brocock Air Cartridge System, for example, uses a self-contained "cartridge" roughly the size of a .38 Special cartridge, which contains an air reservoir, valve, and a .22 caliber (5.5 mm) pellet.Examples of BACS airguns converted to firearms, either by drilling the barrel out to fire a .38 Special cartridge or by altering the cylinder to accept .22 caliber cartridges, have been used in a number of crimes.Blank-firing guns can also be converted by adding a barrel, although the low-quality alloys used for cheaper blank-firing guns may break with the pressures and stresses of a real bullet being fired.