This is the second version of the Gravity Gun. With this cannon you can raise / lower or remove / zoom in on a person, a car, etc ... You can shoot a person in the air, but he will continue to live until you lower him to the ground. You can even burn or boil a person or blow up a car.
To get a cannon, press Ctrl+F to take a person or something else, aim at it and press F, to weld a person, press and hold F, to zoom in/out, scroll the mouse wheel