No advance visa is required for Namibia, just ensure your passport is valid for more than 6 months after date of return and that your time period you are allowed to stay in Namibia as indicated on your arrival form covers at least the time of the hunting safari.
Please bring along any licenses you have for your gun. Upon arrival you will be issued with a temporary import permit for the gun. You have to declare the amount of ammunition upon arrival and departure. Maximum hundred rounds per rifle is allowed and must be packed separately to the gun, i.e. in your checked luggage.