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

Weapon modifications
Extended mag Bayonet Full auto receiver Short barrel

Variants
Combat Rifle

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 straight ahead from the entrance, next to a lit lantern. It's also found during the Synth Retention quest (during The Institute quest-line), immediately after the cut-scene with X6-88.

Behind the scenes
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". In any event, the name is spelled incorrectly : "terribles" is in the plural, while the noun and the determiner are in the singular.

Furthermore, the humor contained in the weapon's name is lost in translation as the word "terrible" in French never refers to something of poor quality or make as in English. It means "terrifying" or "very violent" (as in an earthquake) : in this context, it only means "terrifying", as the second meaning requires the word be applied to the effect rather than the cause.