The word Cannon refers to land based artillery with it’s projectiles being called cannonballs.
The word Gun refers to ship based artillery with it’s projectiles being called roundshot.
Quite often these terms get interchanged, a cannon is a gun and vice versa. Cannon balls rarely get called roundshot apart from on a ship.
Throughout the pages of this site, the words guns and cannons will inadvertently be interchanged. Mainly due to the fact that the word gun, in common usage, could refer to any gun from a small arm to a large land based weapon. Whereas the word cannon instantly creates the right image in your mind.