On the Garand catastrophic failure above there is a key piece of evidence missing from the picture. There are seven cases in the pic, where is the 8th case that was in the chamber?
I suspect if it were available this is what would be seen.
This round was loaded for Greek Army by Brit firm in the 1950s. It was the 17th round fired by friend in his M1 Garand about 8 months back and the same thing happened to his rifle.
This is known as a case head failure. Note the primer shows no indication of excessive pressure. It is simply a faulty case head referred to in our ordnance publications as a L or M split.
Same case opposite side showing "L" splits. "M" splits go through case head from the center to through the rim.
A flaw in the brass could cause this or going through the stress relief line upside down where the flames soften the head is guaranteed to do it to you.
At Frankford this was estimated based on the number of rounds loaded and blown rifles received to occur about every 30 million rounds. About once every ten days at 3 million rounds a day.
In the business we lovingly referred to once fired cases as "proof fired" as those we know were correctly fabricated and were safe to shoot again. This is just the chance you take with new unfired cases.
On high pressure cases you will have a case with an enlarged primer pocket and the case will often be almost welded into chamber and takes considerable force to remove. High pressure rounds tend to cause barrel failures about four inches in front of the receiver.
I did a high pressure investigation where there was an estimated 90,000lb pressure on a case. The case was intact inside the chamber but the primer pocket looked like a 50 cal BMG primer would go in just fine.
To remove the case required cutting a rod about 1" longer out the muzzle and placing barreled action in a floor mounted arbor press with 4" square ram with the receiver on the floor supported by piece of plywood and the ram eased down applying vertical force down the rod onto the web area of the case.
I pulled on the five foot long handle and it did not move. I grabbed hold of bar with both hands and as my feet were just leaving the floor it broke loose out of the chamber. At that time I weighed 215 lbs.
This was caused by referring to a Frankford Arsenal publication for the proof load and the published info called for 4198. The drawing was pulled and the drawing called for the same load of 4895 thusly someone wrote in the wrong data when the FA PAM was published. The case was previously fired so in that instance we knew the complete history.
Most military cases US loaded will show a blue tint around the case mouth and to about 1/4" below the shoulder where it has been stress relieved prior to loading. All new military ammo should be examined to make sure the blue is on the pointed end and not the case head or the above will be the result.
On those cases that have been cleaned (commercial) it is impossible to tell which ones go through upside down and if the inspector doesn't catch it you will find it.