The description for Heavy Armor Training (SWAT level 5 perk) states:
While you have body armor, you only take Health damage from sonic attacks and clots can't grab you. You begin each game with 50% Armor.
However, the only thing that does "sonic attacks" are Sirens who have only done health (and not armor) damage since Killing Floor 1. What exactly does this perk do in terms of damage prevention?
Best Answer
After playing around with the SWAT class for a while and then playing with a without perks, apparently the wording of the perk is misleading.
does not mean sonic attacks don't deal armor damage anymore, as they have never done armor damage. What this really means is that the only source of Health damage is from sonic attacks while you have armor on.
Thus, what Heavy Armor Training does is give you 50% armor to begin a game with. With armor on, you will be unable to be grabbed by Clots and you will not begin to take Health damage until you no longer have armor on, with the exceptions being Sirens' sonic attacks (and fall damage).
Others have been able to confirm this behavior. See this thread: