most if not all the barrels for the golani made by IDF were 5.56 chambered, same goes for all the AM barrels for them i have run across.
when your dealing with bottom dollar builders like CAI thought they may have scrounged up some rejects form somewhere.
what i suspect is the firing pin was stuck, these are known for very rough build quality. probably the round tried to hang up on the ramp and the stuck pin set it off.
shrapnel from the case probably stuck the bolt wen it closed up.
it probably didn't pick up a round cause the mag was flying down away from the bolt during the incident. though i wouldn't bet my life on it.
this is just another case of inexperienced operator and buggie, slap happy low quality gun building IMHO.
some one with experience would have caught these issues before it went "boom".
one good thing about full-auto stuff being so hard to get is it does prevent a lot of firearms "k-booms"!
just imagine a bunch of "greenines" out there with bottom dollar CAI made full auto stuff firing dubious quality ammo?--scary thought!!!