So weapons tiers and damage values are actually calculated off their their real stats. Without going into too much detail we basically took the muzzle velocity and the mass of the real weapon round to figure out its energy and then applied the square root of that number to create the actual damage. We've been adjusting here or there over the last few years but overall its largely using the real numbers as a base. We also have a damage falloff curve based on real projectiles which is another key factor to take into account.
As for mitigation, the key thing to know is its a multiplier against the incoming dmg. Keep in mind the assault can stack the team passive with his core armor which gives that "heavy armor" feel. The other stat to pay attention to is toughness found on inserts. Its a flat subtraction per incoming bullet. It might seem like it doesn't do much but if you do some simple math for shotguns per pellet or smgs you'll see that stacking toughness inserts can be the difference between life and death. However, toughness doesn't do squat against a sniper round. If your having sniper problems I suggest stacking health and crit mitigation.
This is why it becomes difficult to display exactly how tough an enemy is when you are making your shot. Its something we've been thinking about for awhile but haven't found a clear solution for. I like to think GRO is treading new ground when it comes to mixing these kinds of customizations and strategies into the gameplay but it also makes it challenging to provide feedback for them all as well.