The actor also has 2 hands, 2 feet (best used to evade with), a head, etc.
You illustrate an excellent point regarding those who carry a handgun and are not prepared to use anything else. Not a wise idea.
It is not a reasonable belief that you must use a gun if that is the only weapon you possess and the use of deadly force would not otherwise be necessary.
You have to be able to articulate reasonable explanation, which would include that you exhausted other options (if any were available, and if they were not, then you must be able to articulate why). You can kill someone or cause great bodily harm with a single punch or kick to the right place. What if your attacker is well trained in hand to hand combat? Are you supposed to guess? So if anything is going to escalate to physical confrontation, you better have your gun drawn before they land any blows.