I do big batches of 9mm.  Like 5-8k at a time.  I always inspect the brass after cleaning to cull any that are cracked or anything else is to messed up.

Anyways occasionally a .380 gets by which I always catch on the 650 before it gets anywhere.  Yesterday I saw a case that was cracked from the mouth to about the midpoint of the case. I saw it in the first station.  I thought to myself I am going to run it through just to see what it looks like/what happens for educational purposes. I thought at the powder through expander it would split open more making it obvious but I do not bell my cases very much for jacketed bullets.  It did not really open a noticeable amount.  Anyways I ran it through every station.  I pulled the round out and you could barely see the crack anymore and it was a big long crack.  I think maybe crimping it made it less noticeable.

I immediately pulled the bullet etc.  What would have happened though if I hadn't caught that?  Would it have been a probable kaboom?

The chamber holds the pressure, not the case.  Shooting ammo with cracked cases is no big deal.

Shot many a 38, 357 and 44 magnum with cracked case sidewalls after firing.   Usually just a result of many firings.  And like was stated above, the chamber contains the pressure, the case is more or less a gasket.

That said, I inspect as I'm sizing and I inspect finished rounds.  Anything like that I set aside and pull down.

Years ago I bought a shitload of eastern european 7.62x25 surplus ammo. So many cracked necks. Still fired fine (yeah, I am that guy). Though cracks are never good the case usually seal just above the case web. Look at a swollen case and you will see the line.
It is still a good idea to crush and toss out any cracked cases you might run across.

I save them and shoot them in practice.  Never had a problem.

the plunk test will catch the ones with split cases but of course, if catch them earlier the better.
Welcome to doing Big Batches

Thanks for the replies.  I just have never to my knowledge fired an already cracked case.  My brass is a mix of brass I often fire through an open bolt machine gun and brass I pick up at the range.  I get quite a few cracked cases running it through the smg.  I always do my best to discard damaged cases so it surprised me.

If you think about it, that case already split during a firing process.  Whether when you fired it or someone else. Not that taking precautions is overrated, but it should ease your mind a little.

I've only had a couple make it all the way through to "finished" round.

Both would easily jam the bullet back into the case when pushed up against the bench nose first.

I'd be afraid the feeding from magazine into the chamber would result in setback with increased chamber pressures.  

Kaboom??

I had a case of re-manufactured 75 gr Hornady 556 ammo.  Almost all of it had cracked necks, and I shot it all, no issues what so ever.

In addition to possible set back when feeding/chambering, you are subjecting your chamber to flame cutting that could cause an irregularity in the surface. The same happens when low charges are used and primer pockets are somewhat looser than normal, you get a ring cut into the bolt face around the firing pin hole.

Toss bad brass when you find it, there's no up side to using defective brass.

Not sure your 9mm would chamber and fire a .380, but you never know.

I would wager that neck tension is almost certainly compromised as well.