Iver Johnson, gunsmith, Maryland

The ring surely looks like it is being peened right at the spot where the extractor nose would strike it when the bolt slammed home. The proud extractor would exacerbate that. The description of a ruptured case indicates exposure of the case, just like ruptures in unsupported pistol chambers. The carbine fired when it shouldn’t have. The bolt was not fully rotated and locked, was able to retract under rearward thrust of the cartridge when it fired and ruptured due to excessive exposed brass. The questionable built-in receiver bridge/firing pin safety failed. We know that many commercial carbine manufacturers could not get the bridge-cut right. The angle of that cut had even been changed in USGI carbines early on to get it right. One should always test the reliability of that safety system. It can be done safely with a primed case. 

If the bolt is rotated just far enough, but not completely, it will fire. The match-up of the hammer nose and the rear bolt-face is suppose to help prevent it from firing, but a matter of tolerance among components, the quality of machining by the manufacturer and wear-n-tear come into play. Usually, it will just break a bolt lug. There is a point when the firing pin tang safety and built in bolt safety “release” and allow the hammer to fall on the pin. This can be just before it is fully rotated and locked. 

And...checking head space on an Iver Johnson is critical to safety. All the ones I have handled or now own left the factory long on head space. Does not necessarily mean it is unsafe to fire, but IJ bolts tend to head space tighter in an IJ carbine than a average GI bolt will. I have/had IJ bolts that will head space 4 to 5 thousandths tighter than the average GI bolt. If you drop a GI bolt into an IJ barreled receiver that is usually much looser than a GI barreled receiver, you could easily get out around 1.310 and into the teens. Some of the commercials claim to be USGI compatible, but they are not, particularly in the area of bolt fitment. 

armonica wrote: Where do you see the crack?

My Inland? It's a 730,000 range. I think that's a first run?

In the GI version there seems to be a lug. In the IJ version it appears it's just part of the receiver. It also appears to be worn back. Maybe they intended that to act like a bearing. Like an old Babbit bearing.

The softer commercial cast bolts surely are prone to peening. A GI hammer will beat them unmercifully.