Being a survival horror game, you will need everything in your disposal if you want to survive in The Callisto Protocol. Every weapon may come in handy in certain situations, however some weapons are missable.
Chapter 3 has the Skunk Gun which is locked inside a room that can be accessed through a ventilation shaft. Here is a guide to find the missable weapon in Chapter 3 of The Callisto Protocol.
All Chapter 3 Missable Weapon Locations – The Callisto Protocol
Chapter 3 contains one missable weapon, the Skunk Gun.
There will be a portion of the chapter where the elevator crashes and you are stuck in a basement filled with monsters. You will notice a locked door that needs a missing fuse in this area.
Continue through the basement until you encounter four enemies that will be guarding a fuse near them. Take them out and grab the fuse. You will need to use it on the fuse box here to progress.
But before you do, return to the locked door from before. Use the fuse here and you will see a ventilation shaft to the left after passing through the door. Climb on and crawl through it.
You will end up in a room with a few resources and the schematic for the Skunk Gun.
It can be found on the table to the right. Now all you have to do is head to a Reforge if you want to craft it.