It's not from Half-Life 2, the gun's model file is just recycled from it. In Murder, the gun is much different. We don't really need anything besides the default gun model.
Half-Life 2's revolver was nothing like this:
The model is from Half Life 2 and the gun in Murder is only different in how it works, not how it looks
The only difference in models is the view model, and that's because Gmod has a combined view model system, so instead of having Gordon arms, you have citizen arms