You would say the most silent in the world. Then he could hope when the deed was done to slip quickly from the window, to wade the moat, and to get away at his leisure. That's understandable.
你一定會說他要的是世界上聲音最小的武器。這樣他才能指望事成以後，很快就穿過窗戶，蹚過護城河，從容不平地逃跑。這是完全可以理解的。