[Sevens]

It's not a dumb question.
It is a dumb idea.

You'll find some of these pieces on public ranges... it's more common to find the 9mm rounds shot from a .40cal and .40's shot from a .45. I would like to hope that it's usually done in ERROR and not on purpose. I think where it happens most often is when two shooters are sharing very similar guns and magazines get mixed or careless loading of the magazines happen, especially if the more novice of the shooters is handling the ammo at that time.

It's not a huge "safety" issue. When the round is smaller, and neither the cartridge case nor the bullet are RESTRICTED in any manner, pressure can only drop with the extra space.

In the case of .380 being discharged in a 9mm, the cartridge case is a wee-bit smaller, so it will fire-form to the chamber and the bullet, being the same diameter, will offer the "proper" level of resistance in the bore. What will most likely happen is that the .380 round will discharge almost exactly as it's designed, and send that bullet out at typical .380 speeds. The 9mm pistol may or may not cycle. The brass will be slightly blown out to the spec of the 9mm chamber. Remember that all .380 ammo built to proper spec runs at a much lower pressure than proper 9mm ammo does.

If you shoot a 9mm in a .40cal, you're running the SAME pressure ammo, but without the resistance of the bullet fitting in to the bore. A .356" 9mm slug will simply fall down a .40cal bore with zero resistance. So when you pull the trigger on that, you'll NEVER reach full pressure because the bullet has no genuine resistance.

.40cal discharge in a .45 pistol is a better example: Here you have ammo that is significantly higher in pressure than the pistol for which it happens to be in. Sounds like disaster... but it's not. The .401" bullet meets no resistance in the .452" bore, so the 35k PSI max never gets reached, or anywhere near, because there's no real resistance to max it happen.

It's like a firecracker. Rip one open and you find just a dusting of propellant. So little, it's almost unbelievable. But if you wad it up TIGHTLY in a tiny space with all that tight paper, it becomes a little pressure-bomb and it'll take your finger off if you let it.

Putting the wrong ammo in any firearm is almost always a genuinely BAD idea and could also lead to other related issues, but it's not always going to be a catastrophic event unless the math adds up. Try it with some bottle neck rifle rounds... where the shell fits but the bullet is LARGER than the bore and now you've got a potential DISASTER on your hands.