Obviously in "the real world" this all can and does vary highly. As I am sure you know already, bullets do weird things. That said in my relatively limited experience, patients who have been shot very quickly sort into "Dead" and "not dead" categories, usually well before they hit the doors. At the end of the day which pile they end up in depends entirely on what anatomical structures are violated by the bullet (or other trauma). The structures that put people in the "dead" pile are heart & great vessels & the brain/upper spinal column. I can also definitely tell you that I've seen people who have had non-life threatening traumatic injuries who required a lot of restraints (of the physical & chemical varieties) to keep them under control and were not obviously under the influence of any drugs.
As a shooter - and maybe Wayne can speak more on this himself - I've always been a fan of the "aim small, miss small" philosophy. Sure, in reality, there is nothing hard or magical about 5.5". However I'd still rather make that my performance baseline so that if I am more likely to "rise to the level of my training" as it were. I think the point about goals (like a 5" circle) isn't that you will always make them, but that they guide you to a level of performance that you find satisfactory. I happen to agree entirely with Wayne's.
Just my 0.02 USD