When we played, we asked, hey, what's ap rounds in a 9mm punch through in real life (or what would it if it was real)? and we applied ap to any armor that applied; same with the other weapons. You've got some 50cal ap sniper rifle you can ignore the armor of that car, probably that even armor granted by that building. Cool.
I always give the narrative the precedent. If someone shoots someone else whose tied to a chair, and they've got armor but not in the face where he's shooting point blank, hey look it! it's now ap! Exactly the same, but opposite example: if you've got a gun that shoots through armor, you can punch through that guy wearing 2-armor, but surround him with 60 other people and shoot randomly into the mix? They keep that 3-armor from the size. Cause... duh? As for the argument that ap is somehow expensive in this game, I lol to that. It's cheap, like everything else, and easy to get. Roughly worth the cost of one serious gun, if the gunlugger's playbook has any say.
I consider the rules of this game to be exceptionally flexible.
That being said, your example has some things I'm not quite okay with. For example, if someone is using a gang as a weapon, that gang deals and takes the harm. If you're in the gang too, cool, the gang, not you, still deals and takes the harm. If the gang gets 2-armor and +2 armor from defenses and +2 armor from size, then ap knocks off 2, maybe 4 depending on the gun and the story in that moment. However, if you've got a guy who was seized from the middle of the gang and the pc ends up targeting specifically him, I'm not giving that guy the gang's armor bonus, just like I'm not inflicting that harm on the gang. Nah, that's harm against that guy, and gets calculated all by itself.
Armor from a gang's size is not armor, it's a resprentation of how many bodies are sharing that pain.
That's my say anyway, do what you want.