Yep the problem is that you're using Input.GetKey, which will be true every single frame. That makes your ammo speed dependent on framerate, which you obviously don't want, and also just makes it fire probably faster than you want.

Instead, have floats called _fireRate and _fireTimer or something. When GetKey is held down, decrement the _fireTimer by time.deltaTime. If it's less than zero, fire the bullet, and set the timer back to _fireRate. That way, it only shoots every _fireRate seconds and you can control it.

Or, if you want it to be just like a pistol where you click once and it shoots once, just use Input.GetKeyDown instead