In addition to the above, self-sealing technology is a thing we have right now. Self Sealing Tires and Fuel Tanks are out there. Won't help with a meter wide gash in the hull, but could easily deal with both micro meteors and .223 rounds.

I should have written cm. It would take 7 hours to depressurize the 930m^3 volume of the ISS through a 1cm^2 hole...

Also, the defender might decide to depressurise the attacked area on their own to (i) make boarding more difficult and (ii) because it might get depressurised anyways, so why not move the precious air to a safer place beforehand?

One thing worth noting: just because you lose air slowly doesn't mean it's not a problem. It's not a problem on an airplane because you're in atmosphere, and you can get down to ground level quickly and stock up on air again. In space, once you lose air--any amount of air--that air is gone and you're not getting it back without a non-trivial amount of work.

It's not an immediate problem, compared to the space pirate holding the gun that made the hole. My point is that immediate, violent depressurization is what most people expect to happen. But it isn't going to happen. You'd have plenty of time to fix the hole. The air can go, but there's likely days and days or weeks or possibly months of air stored. So that's not that urgent either. Spaceships have large reserves of air in storage, sometimes solid storage like Vika on Mir. No major risk of loss with that

@MauganRa The ability to depressurise arbitrary small sections of a ship on demand requires a lot of seals and vacuum pumps that aren't terribly useful in daily operations - basically you have to turn every section into an airlock.

Spaceships were, are, and will be pressurised with pure or almost pure oxygen. Normal atmosphere has pressure 101 kPa, of which 21% is oxygen. Well, turns out you can just discard all the other compounds and leave just 21 kPa of oxygen and it will have almost no effect except the walls will only have to hold 21 kPa rather than 101 kPa. Only water now boils at mere 61°C, but that's high enough to keep your body from drying out.

… The Apollo fire was caused by design mistake where they replaced the atmosphere with pure oxygen at sea level pressure of 101 kPa already before take-off. In designs that start with normal air and only replace it with pure oxygen as the pressure drops the fire risk is manageable. And the oxygen toxicity also depends on the partial pressure. Oxygen is irritating from about 100 kPa partial pressure for long (many hours) exposure and acutely poisonous at some multiple of that. Breathing pure oxygen at 20 kPa is fine, for any time.

@JanHudec What are the physiological effects of only having a fifth of normal air pressure?

The partial pressure is pretty important in that regard. If its pure oxygen you'll be ok, but if its regular air that just dropped to that pressure you're going to faint. You'll have a major risk of the bends if the pressure drops a lot over a short time.

@Innovine, decompression sickness occurs if the pressure drops by more than some 300 kPa. I've never heard of anybody getting decompression sickness in aircraft depressurization incident and those are pretty close to what we are talking about here, because the pressure at 40,000 ft is 19 kPa, so basically what is going to be in the spaceship (oxygen masks can't provide the oxygen at higher than ambient pressure without full pressure suite and pressure suites are not worn in passenger aircraft, which regularly fly at that altitude)

Another issue with this shielding, if it were impacted directly, is it might bounce off the bullet or part of it and cause unwanted damage elsewhere. This could be solved by padding the inside of the craft.

@JanHudec astronauts preparing for a spacewalk on the ISS "camp out", breathing low-nitrogen air for many hours prior to a spacewalk, to avoid decompression sickness. The space suit is pressurized at 36kPa, pure oxygen, and the ISS at 100kPa, standard air mix. If there were no danger, please explain why they bother with this procedure. By the way, airplanes are only pressurized to ~75kPa, and can make a rapid descent to repressurize the occupants.

@Random832 Present-day also are divided into sections by watertight bulwarks. Most safety and emergency features are not that terribly useful in day-to-day operation. And all those minor airlocks and doors just have to hold out long enough until the threat to the outer sections is over.

"humans can still remain conscious in a vacuum for up to 30 seconds" but how much bodily damage would they sustain in those 30 seconds?

@weston Not much, sore ears were reported by Jim Le Blanc, who passed out in a vacuum chamber after about 30 seconds in vacuum when his pressure suit failed during a test. The chamber was quickly repressurized (which was probably what hurt his ears) and he regained consciousness and walked out of there basically fine. A video of the incident is on youtube.

And was it a total failure of the suit, or just a loss of pressure? I.e. was he totally subjected to the vacuum?

@Innovine "You'll have a major risk of the bends if the pressure drops a lot over a short time." - the actual pressure, or the nitrogen partial pressure?

Sounds like you will be out in 2 seconds flat if you try to breathe vacuum because of water vapor bubbles that will (reversibly) cause your heart to try to pump gas: See this post "We've seen gas bubbles form in the veins and heart of experimental animals, we've seen experimental animals expand like balloons to twice their normal size and we've measured the composition of the gas, and we've reproduced the effect by decompressing the hands of human volunteers. We know what happens."