I am not sure that the poster is genuine. "Thousands of autopsies a year".... that sounds more like Rio de Janeiro or Mogadishu or something than Atlanta. Looking at crime statistic, the greater Atlanta metro area has never seen more than 300 homicides a year, and that was back in the late 80s, early 90s. The data does not include suicides, but still, that is way more gun deaths than that area sees. Also, most homicides are not the results of gun battles, and the stereotypical account of gang-bangers emptying clips of 9mm with their weapons held sideways above their heads sounds more like a personal fantasy than a scientific account.
A former student of mine was a medical examiner at the morgue (Hamilton County Coroner's office) in Cincinnati, Ohio before he went back to school. From his description of the operation there, on who examines bodies, on what types of analyses are run, etc. this guy's account does not match up. Unless the writer was counting routine toxicology measurements (such as BAC in a car crash), the volume of autopsies he is talking about is absurdly high.
What he says about terminal ballistics is somewhat plausible, but it strikes me as being made up to justify his choice of caliber. Maybe I am just suspicious, but you have to be on the internet where it can be very difficult to fact-check.