Okey I've tested it thoroughly now.
It seems that this is indeed an existing bug. Bossk can deal MASSIVE damage, even without headshots. I've tested it in arcade and was able to reproduce it in multiplayer as well.
According to my testing and this post here, Bossk can deal about 150 damage max when his gun is fully charged.
How I performed the shots (I play on PC).
1) Charge up the gun by zooming in.
2) Once zoomed fully in, aim to an enemy
3) Release the right mouse button (the zoom) and press simultaneously down the left mouse button (the shoot).
4) Extra damage profit.
Here is the video clearly showing the extra damage:
So if you perform a headshot and get the multiplier on top of this damage, I do believe you can one-shot other enemy heroes....
This truly need some solution, @EA_Ataashi
I was sure I had replied before that I managed to trigger this bug too. Well I did, and now I tried it after the Chosen One update (02/27/19) and it's still in the game.
