The worst AR Kaboom I've ever seen. The shooter came out of it with minor injuries, but his Cavalry Arms receiver, barrel extention, and bolt carrier group were lunch.
Scroll down to the bottom for pictures.
I've read most of the information that is there, and the cause was confirmed as a mixture of pistol and rifle powder. That will do it.
The mixing of gun powders is exactly why savy reloaders only have one powder out at a time. I keep mine locked up in a cabinet, and only bring out the one that I'm going to be reloading for that specific cartridge at that time. I learned that lesson the hard way once when I was loading .308 Winchester and had Hodgdon H335 and Accurate Arms 2460 on the bench. After a lunch break or something I came back and couldn't remember which one I was using, so I ended up pulling about a dozen bullets and started over. I like my face right where it is.