This is long and we'll thought out, so koodos. That said, I disagree with you though from the first line. You're entire thesis, is based on the premise that the exchange begins when the first shot is fired. In fact on Destiny, the exchange begins when you see the red bar of the enemy on your radar, or spot their toon in the distance and begin manouvering towards them. It's about map placemenr, reading their position, ambushing them or being ambushed. Sure OHK weapons can be abused but a skilled user will always win out over an unskilled one with the last clutch headshot being the first time someone pulls the trigger, but the exchange itself began before that....long before that when they began to engage.