You mention stories about amazing feats people accomplish while mortally wounded. There are stories (true or not) of say, mothers lifting cars off of children after accidents... this is the exception, not the normal "Realistic" result. The same applies to everything else, the stories you mention are so incredible because they are so unusual. Also, I never said every gun shot was deadly either, I said it makes things much more complicated. Walking/moving on a leg that just got shot is similiar to attempting to walk on a leg that just got broken. Attempting to fire with an arm with a gun shot is definitey going to effect your aim, these disadvantages are what get you killed, I was under the impression that you wanted gun fights to be more survivable, it wasn't particularly clear that you simply wanted to make the player take longer to die. It's possible, but your not going to be doing jumping jacks or sprinting to cover. Getting hit in the chest while wearing a vest will still knock you down, take your wind and possibly break some ribs. Also, if you got hit, you must not have been in good enough cover... who ever is shooting at you now has a much easier target.
The problem I am having is that a shot through the heart, note that the game only counts the ventricles as heart shots and doesn't count shots to the atrium because they're totally different, while not an instant kill most of the time, is unsurvivable even with medical attention due to extremely heavy DOT that lasts an impossibly long time and a special constant damage effect (if large enough) that never goes away. This is realistic, being shot through the heart in any situation you'd find in game would be an absolute death sentence. (The circumstances in which you could actually survive such a wound are squarely in the "not goanna happen" category anyway, and they're impossible to replicate in-game.) The problem is, the heart is not an especially hard target to hit and such a rapid death is a big deal in a game where it's normally rare for a single gunshot wound to kill you and dying from anything tends to take rather a long time.
Here, you explicitly state that you think a hit in the heart is the problem EVEN THOUGH it doesn't always kill you instantly... indicating that any other fatal shot that has similiar results (i.e death) would be an issue. You say in your game it should be "rare for a single gunshot wound to kill you", but without the right kind of attention it is not "rare" for a single round to the chest to kill a person.
A statistic from www.trama.org:
"For penetrating thoracic injury the survival rate is fairly uniform at 18-33%, with stab wounds having a far greater chance of survival than gunshot wounds." http://www.trauma.org/archive/thoracic/EDTrationale.html
The fact that you would prefer a gun shot wound to take a long time to kill you, would generally lead someone to believe that you would attempt to give the player options they could take to avoid getting killed after being shot... not many game designers look for ways to make more of the time players are playing their game have no "winning moves".
You then say "many enemies in the game also use guns"...enemies that use guns tend to be humanoid... and if guns are common it generally leads one to believe that gun fights will be common. If gun fights are to be common then the player seems to be expected to be able to survive gun fights commonly. In order to answer this game "realistically" as you seem to have wanted we can only fall upon knowledge of other instances where there are many enemies often carrying guns in real life... which oddly enough is a pretty good description of a war.
I did actually realize that my first post, while I had hoped to be helpful didn't actually address the issue concerning the difficulties of aiming... which is precisely why I added the second post which dealt exclusively with the many variables that affect accuracy. I do actually have experience with weapons, I was combat ops in Iraq for two separate years. I have been trained on the maintenance and use of an array weapons... granted I was a General Issue Joe and not the super star spec ops... I still feel the super human abilities your ascribing to the spec ops guys sounds more like holly wood fantasy then the "do what works" reality.
You should try to control your temper. It's rude to treat people trying to help you the way you do.