Why use the gun? If you have the bow (more silent and it kill very well) why use the gun, who do a lot of loud?

Accuracy? Stopping power? More plentiful ammunition? Because you feel like causing a ruckus?

Basically that.

Ok, ok xD
Another dude, you can blow up a leg of an enemy with the gun?

You can shoot him in the leg yes and he will be stunned for a quick moment.