A crew-served weapon cannot, in practice, be operated by one person. When all preparations have been made, an individual might be able to fire it, but not reload or move it. Some weapons, such as light machine guns, can be used briefly as an individual weapon, but a single operator will soon become fatigued, run out of ammunition, etc. While Hollywood has been known to portray action heroes, for example, firing a .50 caliber machine gun held in their hands, a real human being attempting to do so would find himself flat on his back.
Some weapons, such as guided missiles with nuclear warheads, are deliberately made, for safety reasons, so that they cannot be launched by one person. A typical intercontinental ballistic missile has to have a minimum of two keys turned essentially at the same time, but they are too far apart for any person to reach.