I feel somewhat silly asking this, but the Wild Talents 2E rulebook keeps likening heavy armor to a "really thick eggshell" that is "all or nothing." I was under the impression for quite a while that Heavy Armor and Interfering Defends powers really were all-or-nothing. For example, I thought that if someone had 5 heavy armor, and an attack of width 5 hit it, nothing would get through, but if an attack of width 7 hit it, the person wearing the armor would take the full seven damage. Reading through again, however, I can't find anything to support this, so it makes me think that a person wearing heavy armor 5 hit with an attack of width 7 would take 2 damage. Which interpretation is correct?
In Wild Talents, there are two flavors of armor. Light Armor (LAR) and Heavy Armor (HAR).
Light Armor does two things. First it reduces all Shock damage from the attack to 1. Then it transforms a number of killing damage from the attack equivalent to its rating to shock. The remainder is taken as killing damage.
For example, if you have LAR 3, and get hit with an attack that does 6 shock and 6 killing, you'd take a grand 1 shock from the 6 shock, then 3 more from the LAR 3 converting 3 of the killing to shock for a total of 4 shock. Then you'd also take the remainder killing for 3 killing.
Heavy Armor is a bit different. Heavy armor reduces the width of the attack roll before damage is figured. The fact that it reduces width is where the "all-or-nothing" designation comes in. If the width is reduced to 0 or less, you do no damage. But if the width is greater than 0 after the HAR is taken into account, you add the pluses from the damage, and they are not reduced at all.
So, if you have HAR 2, and get hit with an attack from a machine gun that does W+2K, and the roll is Width 2, the +2 doesn't matter. The attack is reduced to Width 0, and doesn't make it through. If the roll is Width 3, only one point gets through- but all of the pluses count at full strength, unreduced by the armor.
So, the short of it is, you're right, the damage is reduced by the armor. But it's the Width that is reduced- not the final damage.