The average armed encounter lasts only two or three rounds; the person with a second weapon, who doesn't have to reload, is at an advantage. A concealed weapon can be used when an officer is disarmed or his primary weapon malfunctions. The off-duty weapon, concealed and usually smaller than the official service revolver, is used when officers are on 24-hour call, may run into hostile people they have previously arrested, or live in high-risk areas. All auxiliary weapons used by police officers must conform with their department policies and be issued or approved by the agency. Agency policies also cover acceptable ammunition and often mandate qualification courses. Back-up guns should be smaller versions of the service gun, so that the officer is accustomed to its feel and has the correct ammunition. Some back-ups can be matched to the duty gun to allow quick ammunition exchange. Auto pistols are generally preferred over revolvers for back-ups as they are flatter, more concealable, and carry more ammunition. Despite their lesser cartridge power and range, sub-caliber guns are useful when the officer is limited to concealing his weapon in an ankle holster. Police agency policies often dictate the mode of carry, specifying shoulder, belt, or ankle holsters, or pocket carry.