Not Legal Advice: What we found is that police code 413 is commonly used to report a person with a gun, 413A is for person with a knife, while 413B is for person with other deadly weapon.
Please note that some state law enforcers may use a different police radio communication signal from another. Some police codes are common to various police departments, while others are exclusive to a particular department or station.
For those outside of law enforcement, listening to a conversation between two police officers - or just listening to a police radio - may seem like you're trying to decipher an entirely different language. That's why if you're interested in pursuing a career in law enforcement, you will need to know police lingo so you can learn to speak like a cop.
Check out some tips on how to talk like a cop at About.com.