Rules regarding shooting are player made and enforced. New players are strongly advised to seek guidance from their crew leaders regarding the use of their gun before attempting to wack. It is extremely likely that "wrongful" shootings will be punished by death.
In order to wack an account three ranks above or below you you will need to Request Auth.
Generally, Non Player Characters (NPC), Made Inactives (MIA), and Required Inactives (RIA) are the only accounts you should be shooting at unless otherwise ordered to by your leader or member of the crew's upper structure.
A Wack will result in one of three possibilities: "Success!" (Kill), "Falure!" (Miss), or a Wackback (Miss + Bad Consequences). There are three kinds of wackback: Gun Loss (you have to re-acquire a gun), Jail Time (you are given an extended jail sentence), and Death (your character dies). For any given shot, you have a small chance of wackback.
Always make sure you know your crew's policies in regards to shooting before you shoot.