sorry, i meant Drachen_Jager

Siftbot is right about the 90% part. I would guess that the shooter is a reloader. Squib rounds are common with reloads. the first charge is not strong enough to send the round downrange and it jams in the barrel; the second round comes through and the added pressure either pushes the squib round out, or the pressure exits via the barrel. the bolt is locked in the breech and the pressure needs to go somewhere. the barrel is the weaker of the two and it goes, rather that driving a broken bolt through the shooters face.