Re: KABOOM !
First of all, I hope you are a right handed shooter and you came out of this unscathed.
Second, and I'm sure I won't be the only person to say this, but it appears that you have had a case head separation. That indicates there are other forces at work here, other than a bad barrel extension.
Is the bore clear of obstructions? Was it prior to firing?
Is this a first time fire of the rifle?
Honestly, IMO, it looks like it was either over-pressurized, not headspaced correctly, or there is a barrel obstruction.