As a guess, a hole somewhat larger than a softball is probably going to be noticeable in terms of lost cabin pressure, though it’s going to depend on altitude and the position of the hole. At low enough altitude, a hole through skin and nonstructural components is just going to be an annoyance, not a threat in terms of loss of consciousness. A quantitative analysis would have to take into account a lot of variables and is more than I care to get into here (though if you submit it as a Mailbag question perhaps someone will take up the challenge) but here is an article about a loss of pressure incident resulting from a moderate size hole–on the scale of what I’m talking about–from last year.
I believe the pressurization system on the cabin of commerical airliners is separate from the pressurization system for the main cabin to help prevent loss of pilot control due to depressurization but I’d have to check on that. Breathing from the demand oxygen system that deploys automatically upon depressurization should allow you to remain conscious at any altitude, hence why it is recommended that you first put on your own mask and then assist others.
Looking at the link that Stranger provided, it seems that violating the integrity of the aircraft’s hull is apparantly considered sufficient cause for the stresses on the cabin to lead to expanding the original flaw. Now a circle would be the best shape for a hole in metal to be, but I’m not sure a gunshot would leave a clean circle. (In fact I doubt it strongly.) Based on the way that cracks can propogate and the assumptions in this article, I’d not be surprised if the hole from a firearm could expand to become a problem for maintaining cabin pressure.
I know it’s not an automatically fatal failure, as you say the oxygen system does allow for people to remain conscious, if they use them properly. But I’m afraid I’m still going to think that holes in the hull are a bad thing because of what it may do to cabin pressure.
Possibly, if fired nearly parallel to the skin so that it made an elongated hole or scored the skin (as I’m assuming the baggage truck did), but I’m assuming that the bullet punches through the skin at more than a 30 degree angle, leaving a relatively clean, non-crack propagating oblong slot with dimensions roughly 2:1 to the nominal bullet diameter. (The aluminum aircraft skin will provide barely more resistance than thick paper to a fast moving bullet and shouldn’t cause the bullet to deform measruably.) If the bullet is a ricochet, however, it might be tumbling and “keyhole” through the skin, making a more irregular hole that might be prone to crack initiation.
If it’s visibly spreading (or just about to) it’s probably going to rip all at once, like a zipper. But if he drops before it starts to go and normalizes pressure then there’s no impulse (other than normal hull vibration and incidental stresses) to cause it to spread.