Unknown Armies is the best highly-simple damage system I can think of that still fits the typical RPG structure. Attacking is a percentile roll, try to roll beneath your skill.
If a firearm, damage is the result of that roll up to a cap based on the general size of the bullet.
If a melee weapon, damage is the dice added together with some extra damage for each of "Is it heavy? Is it two handed? Is it penetrating?" that you can say "yes" too.
Players never know their hitpoint total; that's tracked by the GM and they only know what the injury feels like. This lets the GM describe the same amount of damage very differently based on the character; while the ex-special forces guy might get "you feel an impact on your arm, a quick flex shows the limb still functions fine and the amount of blood isn't too much so you can look at it after the bullets stop flying" while the nerotic acedemic might be told "OH MY GOD YOUVE BEEN SHOT AND THERE IS BLOOD EVERYWHERE!"
In terms of maths the system is super-fast; whenever you call for a roll a roll it's percentile so there's no fussing around to figure out what dice to pick up and the success/fail check is a straight lower/higher comparison.
It's not something that would work if combat was the focus of the game or if you wanted the typical low-lethality of RPGs, but if you want a meth-head with a .38 revolver to actually scare your PCs when he tries to mug them it's perfect.