When it went off, the round sounded like it was a light load or something. The slide didn't cycle and we were concerned about the round staying in the chamber. I picked up the brass and walked over to the table and I waited about 20 seconds, cleared it, pulled the trigger, disassembled, and pulled out the barrel (which was clear). I looked at the brass and it was quite distorted. Someone suddenly said "Hey... that's a 9mm!!". (*facepalm*) Looks like a 9mm round must have fallen out of my hand when I was trying to load my K9 and fell onto the .40 tray.
When the round went off, it expanded to fill the chamber.



Looks like it'll go off, but then you have to rack to get the next one. I don't recommend it at all.
Brian, thanks for letting us out there. I can't wait to do it again!
When you do an update on the AR forums, hit us w/ a link here so I know where to go! =-]