Armor based location damage reduction FPS

How would i do a location based damage reduction with armor and even have an armor penetration system?

Example: i have a gun that does 13 damage per shot. I am hitting a target that has 11 HP and a natural damage resistance of 3. So far i can destroy target with two hits. All of this is already done and coded what i cant figure out would be once i add armor. Say i add sholder armor to targets arm and create a hit box over said armor. That armor further reduces that 13 damage by 4. Now that shot of 13 does 9 damage that passes to target and the natural damage resistance reduction is an additional 3. Total damage is 6, needing 2 shots to destroy target. Now the last part is what i need help with. How do i have it to where if i shoot sholder armor, it further reduces damage, but if shoot rest of the arm it doesn’t. Also, (to make it more complicated) if said weapon does 2 extra damage and 2 penatration on a “high setting” how would I get that 2 pen to negate 2 armor but not the natural damage resistance on rest of arm.

Ist it easy and already know whow to do it and probably over thinking it? Ya maybe but help is appreciated.

What is this?

Might have put in wrong place. Looks like i wanted character and animation? If thats what you mean. Or was my rambling not comprehensiveable lol