Or then the guns have built-in sensors that figure out potential targets, and give a little buzz against the user's hand, or other such signal, at each target the gun sweeps past. The user can then press a button to lock the gun to that target - and afterwards, the gun will fire at that target no matter where it is being pointed. Much like the "repeat" button on Gary Oldman's gun in Fifth Element
There are pros and cons to both approaches. You don't always want to look at your target when firing, for example...