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"

its based on a 1897 (trench grade) Winchester shotgun