[Hummer]

When I worked for the Army Small Cal Lab I investigated this exact thing several times.

Several things may be in play here.

Question 1. Was this a new factory round on first firing?

Question 2. Was this a reload you actually fired previously or brass someone gave you or you found?

It appears that there is no blue tint on the neck/shoulder best I can tell from pics.

In the older loading lines a case would go through the annealing line of gas flames before being loaded and the neck/shoulder is light blue. In the older lines a case would go down through the flames UPSIDE DOWN so the case head got the heat and neck and shoulder would not.

Thus the case head is dead soft.

If you bought the brass and don't know the history chances are good it may have been in a fire (grass fire/ trash can etc) which would soften the whole case. Thus buying shiny brass in a bag at a gun show you don't really know the history of the case.

As well some jerk may have heated the head, tumbled cleaned it to make it pretty and let you hold it. I don't use fired cases unless I know the history.

It was estimated by ammo engineering that one case would go through the flames upside down about every 30 million rounds.

I had an investigation which was much more severe than yours. Upper and lower were bulged out, magazine went into troops forearm and had to be removed surgically. On that investigation I took it to industrial X ray at Picatinny Arsenal and we took a pic and found a bullet nose to nose with a AR cleaning rod jag and patch on it where it came loose on the range when the barrel was "rodded" after firing.

Rifle was blown to hell and no bulge in barrel.

Cases sustain plastic deformation/failure around 90,000 psi if they have not had their heads subjected to heat.

From the looks of the primer I would say you did not have excessive pressure.

The good news is I have never heard of a bolt lug failure in a M16 with MILSPEC parts.

Glad you were not injured. Lord was looking out for you.