The .380 will fire in the 9mm chamber since the extractor holds the round against the breech face sufficiently for the firing pin to ignite the primer. Upon firing the case rams against the recoil surface of the slide. It is not a wise move given the current difficulties in finding ammunition. Save the .380 for pistols in that caliber.

Just as an aside, a similar situation have more than once occurred when people were firing M-1 Garand rifles in .30-06 and inadvertently loaded 7.62x51mm/.308 caliber ammo. The rounds feed, fire and eject just fine. POI is usually lower on the target than what is normal using standard M-2 ball. There is usually no problems from this particular error. The casing shoulder is blown forward fire-forming to the chamber. Normally the fired casing ejects and the rifle feeds the next round. But of course it is best to fire any firearm with the correct ammunition.

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