I would not worry too much about it.
If weapons are allowed in an airplane's cabin, it is obviously because the people who have them, present no security risk, and are authorized to have them on board during the flight. Could be police officers as an example. Ammunition present NO danger with cabin decompression. Of course, a fire like you mention could present danger, but such fire (cabin overhead lockers) would in itself present other dangers to you and the airplane than the ammunition. Extreme situation you mention.
xxx
A pistol, in itself, even with a magazine full of bullets, presents NO danger whatsoever, provided that a bullet is NOT chambered (by "sliding" the action). Removing the magazine from the pistol is not added security. Those "cowboys" or "Dirty Harry" cops who carry a pistol with a bullet chambered with the excuse that the "safety" is ON are always dangerous. They might shoot theirselves in the foot. Same with a revolver (6 shot "wheel gun"). For these, it is better to remove the bullets from the weapon.
xxx
Do not worry either about a handgun bullet fired in the cabin by accident, piercing the fuselage. Chances are that little damage will be sustained, unless hitting certain aircraft components. Would simply be an extra air leak, like there are a few in airplanes.
xxx
I am an occasional "skeet and trap" shooter, and have, at times, taken my two guns as checked baggage, with ammunition. Other than declaring them as required, no precautions are needed for the ammunition in checked baggage.
xxx
If your name is Osama, please remove bullets from your AK-47...
And you are not welcome in my plane.
Happy contrails