1098cya wrote about his Kaboom experience at AR15.com …

From there, we took it apart and used my cleaning rod to push out the broken spent shell case out of the chamber. Needless to say, no more shooting that rifle. We surmised a possible “squib load”, double load. But, if you look closely at the damaged spent shell casing, the end is pretty clean and the rest is fouled. RSO said that the round wasn’t fully chambered.

When the firing pin made contact, about an inch of it was also fouled. The gases went straight from the chamber, down into the magazine well and blew out the bottom of the magazine. You should have seen my face, as I fired off that round and heard and felt something different. Looking down, I saw the bottom of the magazine all blown out and seeing the magazine spring hanging out too!

[ Many thanks to jdun1911 for emailing me the link. ]