To be fair, something I don't think I've seen anyone ever take into account is recoil, which would effectively limit you to a short burst of accurate fire as Skallagrim said, before you couldn't logically be firing with the same accuracy. 

The issue is with the fact that the gun-user has control over if their shots are accurate or not, and with the travel speed and general short preparation time of guns, its almost the equivalent of a force hit at very low tiers, just because it is so much faster than most characters can react.

To sum it up in a nut shell, the reason why guns seem unfair can be explained by a historical example. If you gave me a pistol and set me up against a samurai who had trained his entire life with the katana, and set us about twenty feet apart, I'd still probably kick his ass 9 times out of 10 with my lack of experience or training or skill or anything remotely resembling talent, just by merit of my gun. Melee characters have such an inherent disadvantage against anything other than a single shot weapon (provided it has the capacity to quickly kill them, which it usually does.) that it throws a spanner in the tier system.

The point Tuuj that you keep making about reflex speeds of biological creatures is somewhat pointless though, as magic can easily allow you to circumnavigate those restrictions.