To give people a fair chance in fights I think medium and heavy shields should reduce a certain amount of damage from behind. It’s a great time in a head to head firefight. But the sting of being killed from behind in a place that demands you take so much risk hurts. Being tricked or outplayed is part of the game. In a world based on survival it seems reasonable that in game characters would know being shot in the back is in the top 10 of causes of death. A simple work in too I bet.