I would suggest: You fire a gun. You shoot your target (with the gun).

There is a very subtle difference. He fired the gun is better than he shot the gun.

I would consider them mostly interchangeable. The only difference is that shoot can be used to describe the action of shooting the gun or hitting your target, but fire can only be used to describe the act of shooting the gun. Examples - Good: I shot a gun. Good: I shot the deer. (Meaning, “I shot the deer with the gun”.) Good: I fired the gun. Incorrect: I fired the deer.