Thank you for the answer ! I'm gonna try to find the second secret/optional upgrade (I don't think I've found it yet) ! By the way I'm not sure what the hidden upgrade behind the "eyes door" does to the shotgun.
The weapon upgrade significantly increases rate of fire (no more reload animation). If you need a hint for the other upgrade - the very first room after the entrance to the catacombs contains an inaccessible ammo upgrade with a book behind it. That book has the answer!
