So this ain’t supposed to happen, right?
Nope in fact it is normally quite hard to get that end cap off.
Is that like having the primer back all the way out?
I’m not sure tbh. @bentaz is our resident shotgun advisor. I think it may be a combination of ejector doing its thing and shell maybe not designed for a gas system? It yanked it a bit too hard or maybe it has a bit of oomph and went forth and tried to multiply in a 3" chamber. I’m not sure.
The gas system may be the culprit. I take it your trying out a new toy.
You take it well.
I think its a case separation (been talking to JS offline). If the extractor pulled it off the inertia should have pulled the hull back out of the chamber, but it was pushed forward into the forcing cone and had to be pushed out with a rod.
Correct on summary.
to add, those specific loads ( Clever | Professional Ammunition - Verona (clevervr.com)) did feel (through felt recoil) a tad more oomphy than the rest of the ammo I was testing.
Could it be, my first case head separation 
Is bad head spacing usually the cause with case head separation? I remember you explaining a little about it when I bought the Enfield.
@Tempestman yes bad head spacing is the normal cause. It causes the case to stretch until it becomes so thin that it just breaks in two.
The way shotty shells are made what has happened to JS is just another form of a case trying to stretch.