Equip a gun with a customizable part.
(warning slight spoiler below)
n Castle Dimitrescu, you can obtain this after
. Continue on the linear path afterwards and you will end up in a narrow hallway that leads back into the main area of the castle. In this narrow hallway, there is a small table with a a treasure box on it. Open the box and you will receive a weapon attachment that you can attach to your pistol through the Inventory menu . Achievement should pop once you’ve combined the attachment with the pistol.