Le Fusil Terribles

The Le Fusil Terribles is a unique weapon in Fallout 4.

Characteristics
25% increased damage and limb damage, but has more recoil

Locations
Found on top of a yellow lootable Wooden crate, just as you enter the Captain's Cabin in Libertalia. The lootable Wooden crate is literally straight ahead from the entrance, next to a lit Lantern. It's also inevitably found during the Synth Retention quest (during The Institute quest-line), immediately after the cut-scene with X6-88.

Trivia
The name of the weapon appears to be a rough translation from Fallout 3's unique combat shotgun, The Terrible Shotgun, although the name directly translates to "The Terrible rifle" while shotgun would really translate to "Fusil de Chasse"