Wolfenstein II: The New Colossus

In Wolfenstein 2: The New Colossus, there exist weapon upgrade kits scattered as pseudo-collectibles throughout the various levels of the game. There are roughly 20-ish in total to be found.

Gameranx has an excellent guide I used for this.
https://gameranx.com/features/id/125482/article/wolfenstein-...

If you're a more visual learner YouTuber "randomChievos" has a guide that's helpful as well.