It's unclear whether this harpoon gun was cut from the final game, or whether it's set to star in downloadable content.
The video, below, again from Tyrannicon, shows it off.
You can play with the gun yourself using xxdeathknight72xx's mod. It overrides the Railway Rifle, and even includes a cool reload animation that sees a trail of bubbles come off the harpoon. Xxdeathknight72xx said the harpoon gun appears to have been in the game during beta testing. Meanwhile, a Fallout 4 modder called xxdeathknight72xx discovered a harpoon gun hidden within the game's files. FALLOUT 4 SECRET ROOM WITH ALL ITEMS/WEAPONS/ARMOR! It'll completely and utterly spoil the game for you. To check it out, access the console commands, then type in 'COC QASMOKE'. YouTuber Tyrannicon used the console to access a secret developer room that features every item in the game, including all weapons and armour. The console is a debugging tool in the PC version of Fallout 4. Fallout 4 players are using console commands and mods to uncover cool secrets about the post-apocalyptic role-playing game.