Defender's Harpoon Gun

Defender's Harpoon Gun is a unique weapon featured in the Fallout 4 add-on Far Harbor.

Characteristics
Admiral's Friend is a unique harpoon gun that bears the Penetrating legendary affix, meaning it ignores 30% of the target's damage and energy resistance., but is otherwise identical to its regular counterpart.

Location
It can be obtained from the Mariner as a quest reward for completing Hull Breach 3.