I've picked up several spent cases at our range where some dummy has fired (yes, fired) .380 in a 9mm handgun and 9mm in a .40 S&W handgun. The cases were swelled up like balloons, but the rounds fired, and there was nothing for the rounds to headspace against.
In one case, the guy must have put several magazines of 9mm through his .40 pistol, as there was lots of swelled up 9mm brass laying around. The amazing part was that only one case started to come apart. The rest all held, which probably saved our rocket scientist from losing an eye from flying shrapnel and hot gasses. If the case doesn't swell enough to seal the chamber, all that stuff comes flying back at the shooter.
Several years ago I asked myself the same question concerning headspace in semiauto pistols. Since I've got them in lots of calibers, I decided to do an experiment. For starters, I tried .40 S&W in a 10mm Witness, similar to firing .22 shorts in a .22 Long Rifle chamber. Every round fired without mishap. Then I tried 9x19 in a 9x21 Witness, and again, every round fired without mishap. I then tried 9x19 in a .38 Super Witness, and again all rounds fired without mishap.
My conclusion was that at least in the Witness, the extractor was holding the case against the breach bolt tight enough for the firing pin to detonate the primer. All the cases I tried were the same diameter as the parent round, so there was no danger of the cases overexpanding and ruptering. My sole intent was to see if the cases did actually headspace on the case mouth, and in these particular instances, they did not.
I don't recommend anyone else trying these experiments, as the results in different guns, and with different ammunition, may not be successful, and may actually do harm to the gun and/or shooter. I only post this to shed some light on the original question.
Hope this helps.
Fred