The Blunderbuss is a ranged weapon, used for defending, or in some cases, attacking another territory. It is a powerful weapon and, as a ranged weapon, requires ammunition. Specific to the Blunderbuss is the ammunition, the 'Bullet'. To load the bullets into the gun, simply place them down from your backpack, equip the Blunderbuss and click on the bullets individually.
Unfortunately the blunderbuss doesn't have a cross hair, so you have no indication as to where you are aiming. Also firing itself is erratic and inconsistent, so you shouldn't rely on this weapon to save your life.