I only skimmed through the first page, but thought I would still comment on this, more for the OP than for you, but still it’s a valid point.
If it’s a soldier shooting you with AP Ammo and the Engineer damage buff, would it not take only a few rounds only to the chest to kill most people?
If that’s the case, it wouldn’t be so random and luck and they may not make any headshots at all, they just get a normal damage buff plus another 50% buff (I think thats right, because after the first damage boost buff isn’t the second one only 50% of what it normally is?), and with that they would only need to land far fewer shots on anybody who isn’t a medic/doesn’t have a medic’s buff.
Factor that in with short controlled bursts, which there are PLENTY of people who do this by the way, which makes their aim more accurate than just laying on mouseone it would be very likely to seem to die instantly. Especially if they get the jump on you.

oh well