I don't think it works quite this way . If it did , you'd not be able to inhale with your muscles with all that pressure on your chest.
Most of the voided air space will be taken up by the bag forming to the gun's shape . Any air left in the bag will be shrunk to a space to yield 14 psi air pressure inside to meet outside pressure. If not , the plastic bag would rip , not the stock or barrel collapse .
Chris