The Silenced Pistol is an unlockable sidearm found very rarely in the hands of enemies. It has a magazine size of 13 with a slow fire rate. The silencer stops enemies from hearing Jacket's shots and rushing to his location.

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For some weapons to spawn, the player needs to unlock them first. Weapons can be unlocked by completing chapters. The score obtained from the chapter goes towards unlocking the next weapon. A new weapon is unlocked every 50,000 points until a total of 800,000 points is reached, upon which the final unlockable weapon is unlocked. Weapons are unlocked in the following order:

 

Katana -> Glass Bottle -> Magnum -> Machete -> Beer Can -> Skorpion -> Hammer -> Brick -> Uzi -> Sledgehammer -> Ninja Star -> Dart -> MP5 -> Fireaxe -> Scissors -> Silenced Pistol

Source

Hope that will help it's all I've found.

The Silenced Pistol is an unlockable sidearm found very rarely in the hands of enemies. It has a magazine size of 13 with a slow fire rate. The silencer stops enemies from hearing Jacket's shots and rushing to his location.

Source

For some weapons to spawn, the player needs to unlock them first. Weapons can be unlocked by completing chapters. The score obtained from the chapter goes towards unlocking the next weapon. A new weapon is unlocked every 50,000 points until a total of 800,000 points is reached, upon which the final unlockable weapon is unlocked. Weapons are unlocked in the following order:

Katana -> Glass Bottle -> Magnum -> Machete -> Beer Can -> Skorpion -> Hammer -> Brick -> Uzi -> Sledgehammer -> Ninja Star -> Dart -> MP5 -> Fireaxe -> Scissors -> Silenced Pistol

Source

Hope that will help it's all I've found.