From the look and angle of the break it sure looks like a a defective barrel extension to me but I am no expert. It looks like the extension cut loose and that basically peeled the side of the reciever back as the bolt BC and extension pushed outward. Glad you are safe its good to see when you break down these weapons construction the way they are designed they come apart in a fashion that would minimize injury.Glad you are OK. Did you get any report back from STAG yet on their finding?
Stag blames the ammo the S&B blames the gun hehe. S&B is buying my last 40 boxes i have of there ammo back for $400.00. and Stag built me a Brand new gun for nothing but it was a barrel extension that failed the only question is why it did. Stag calmed it failed do to the Ammo being to hot ..... i do not think you can pack that much powder in to a 5.56 case to do that Stag ses the presser would have to be over 150k psi to break the extension the way it did but the barrel had no obstruction other then the round that was stuck that went boom. I am going to call stag a gen this weak probably to see if it is back from the lab yet.