The cannon has a control panel which allows the cannon to fire at an angle. Whatever you fire is up to you ;)
1.In order for the cannon to be used, BOTH cannon and control station must be
placed, preferably the command station nearby for aiming sake.
2.On the control station, there are 2 switches and a button. The button is used
to fire. The switch on the LEFT is used to aim horizontally, and the one to the RIGHT is
used to aim vertically.
3.There are several ways to reload the cannon. First you can use the
weapon_physcannon to launch the item into it, as long as the item is not breakable.
Secondly, you can build a platform at the cannons straight out position to carry the object