Bullet Builder is a Double Thief Trap Room introduced in Boss Monster: The Next Level. It is noted as being a "cycling" card, a card which activates when a player discards a card.
This card works similarly to Rust Monster Pen, but targets Monster Rooms instead of Trap Rooms. Being able to take -1 damage from a person with a Monster Room oriented dungeon can be a bit bothersome.
Though the -1 damage might seem minimal, this card combos well with Werewolf Den, another cycling card.
The Bullet Builder makes a reference to a couple of Super Mario elements: The Bullet Bill, and the Bullet Bill Launcher.