When you consider the source of the air for pressurization, LP/HP air
bled from the engines' compressors, and note that just one engine or,
on most transports, the APU as a last resort, can supply enough air
for normal operation of the pressurization system, it becomes obvious
several bullet holes will not materially affect cabin altitude. Note
also that there is constant flow through the pressurization/dump
valves; they will normally never be fully closed. In sum, the valves'
normal open area could be compared to the aggregate area of the bullet
holes so that when the p/d valves do fully close trying to maintain
pressure one could calculate the number of bullet holes they could
compensate for. A nice experiment waiting to be accomplished. BTW keep
your seat belt fastened and carry a nice sharp 6H drafting pencil in
your kit.
Walt BJ