Shepherd has crazy physics, do NOT trust the damage displayed. Not a hack, and the gun itself isn't actually doing that much. Yesterday antagging I got shot for over 600 damage and it didn't even wound me as Hans. I've taken 1600 dmg as Hive and was just wounded.
But yeah it's always acted like this, just some kinda weird interaction with the bounce and crit physics of how the shots work, but based on all my play time, I can definitely say it's not doing as much as the "damage received" shows. But it is very chaotic, and either just mildly annoys you, or totally murders you, very little inbetween.