A rib cage is a sealed compartment under a slight negative air pressure, containing tissues which destroying is the desired goal with a projectile penetrating it.
A tank turret is usually a sealed compartment under slight positive air pressure. It contains tissues, I.e. Crewmen whose destruction is the goal of penetrating the compartment with a projectile.
"Shoot low sheriff, I think he's riding a shetland!" B. Wills
