The option is listed as:
F9: Unlock Weapons - before entering a campfire, toggle this option on and
then enter the campfire. Many weapons that were not available
are available to use.
It was created and tested in the SP campaign. We have no idea if it works in any other modes and we wont be modifying or changing the option.
best,
Cal
I actually have to agree. The unlock weapons option doesn't work as described in the SP campaign. Granted, I didn't try to use it until I was 3/4 of the way through the game, but I do have weapons that are only partially completed and other weapons I haven't gotten any parts of yet. None of which became available. Just FYI.
F9: Unlock Weapons - before entering a campfire, toggle this option on and
then enter the campfire. Many weapons that were not available
are available to use.