I'm noticing, in Co-op at least, that shooting enemies through thin objects like doors or shelving units seems to do no damage, or at least extremely reduced damage, with a lot of guns. The MP7, for example, should have no problem maintaining a similar trajectory and lethality through something like sheet metal shelves (happened to me last game). Doors should not prevent more than 10-20% damage from all calibers, including pistols.
I'm not sure how it's handled, but bullets in real life act unpredictably once they've traveled through a surface, even if the material is rather thin and not very dense. Just thought I'd comment. Penetration on the PK machine gun feels like it's working but that's a full size rifle caliber. Smaller calibers don't seem to penetrate very much if at all.
I've noticed the same thing OP mentioned. Sometimes even when I'm using a weapon with one of the highest penetration stats in the game, and shooting through almost see-through fencing, doors, or thin cinder walls, I do almost nothing (other than make the guy shout)
the only weapon in this game with any real piercing is the Technical. And that thing is a fucking RAILGUN. Walls basically don't exist for it.