ShotGuns shoot either Bird Shot, (Pellets) or slugs (Big round balls of doom).
When doing such, you need to take off one barrel and replace it with another, in this instance the bird shot barrel is smooth as to allow the pellets to pour out in one blast.
A slug barrel (Or any rifle actualy) has grooves which turn the bullet in order to make it fly faster, longer.
Sawing off the bird shot barrel does three things, makes the spray wider and chops the hell out of the distance in bird shot, and add a whole bunch of kick.
Pretty much the same for slug only take out the spray.
There you have it.