Ian
Notorious member
Allen, you have it. Some would use the term "tolerance stacking" metaphorically to describe the situation.
Painted jackets have some characteristics which are not very well understood by everyone yet, least of all me. However, I have done a lot of work with them lately and have observed and actually quantified in one instance the comparative changes the coating makes to both bullet pull and engraving force. A chronograph will also demonstrate the effect.
Some observations:
That load should run about 25K psi, less than half the allowable average peak pressure per SAAMI.
So it wasn't an overcharge under NORMAL conditions. But the primer blew. The case head appears worn but mostly intact, though offset somehow but maybe that's the light. General overpressure? Tough to say, but probably so. Let's not forget that LC 5.56 machine gun ammunition case heads are harder than a whore's heart and have very small surface area for excessive chamber pressure to act upon compared to something like a .473" case head. Also, overpressure is relative to the system, and the bolt face simply may not have been up to even a normal, full-pressure load. This load may have spiked enough to compromise the containment system without showing the normal signs of extruded case head or grossly stretched primer pocket. No brass was washed out of the pocket or onto the bolt face, so pressure must not have been over something like 70K psi. So what blew the primer? Ok, first the brass. Worn out and annealed multiple times. Neck probably very thin from being sized up to 6mm from .22. Super-slick bullet seated into it. Around 20% empty space in the case. Primer pops the bullet out easily and immediately, it sticks in the throat for a split second, increasing the case volume and cooling the charge, then the smoldering powder goes "POOF" against a bore obstruction. Due to the small, light bullet, decent case to bore diameter ratio, the super-slick coating, and the shape of the powder kernels, the poof did a lot less damage to the system than it could have, but plenty enough to blow the primer, and the condition of the rifle may have made that worse than it could have been.
Play stupid games, win stupid prizes. I suggest digging out a reloading manual, reading and heeding ALL of the safety warnings and mandatory brass inspections after every firing, and until you understand what you're playing with better, do not deviate from the COMPONENT selections indicated in published load data. I have had a similar accident with a 1911 because I failed to check one important thing after making a slight adjustment to my loading process. Over-confidence is as bad as ignorance.
Painted jackets have some characteristics which are not very well understood by everyone yet, least of all me. However, I have done a lot of work with them lately and have observed and actually quantified in one instance the comparative changes the coating makes to both bullet pull and engraving force. A chronograph will also demonstrate the effect.
Some observations:
That load should run about 25K psi, less than half the allowable average peak pressure per SAAMI.
So it wasn't an overcharge under NORMAL conditions. But the primer blew. The case head appears worn but mostly intact, though offset somehow but maybe that's the light. General overpressure? Tough to say, but probably so. Let's not forget that LC 5.56 machine gun ammunition case heads are harder than a whore's heart and have very small surface area for excessive chamber pressure to act upon compared to something like a .473" case head. Also, overpressure is relative to the system, and the bolt face simply may not have been up to even a normal, full-pressure load. This load may have spiked enough to compromise the containment system without showing the normal signs of extruded case head or grossly stretched primer pocket. No brass was washed out of the pocket or onto the bolt face, so pressure must not have been over something like 70K psi. So what blew the primer? Ok, first the brass. Worn out and annealed multiple times. Neck probably very thin from being sized up to 6mm from .22. Super-slick bullet seated into it. Around 20% empty space in the case. Primer pops the bullet out easily and immediately, it sticks in the throat for a split second, increasing the case volume and cooling the charge, then the smoldering powder goes "POOF" against a bore obstruction. Due to the small, light bullet, decent case to bore diameter ratio, the super-slick coating, and the shape of the powder kernels, the poof did a lot less damage to the system than it could have, but plenty enough to blow the primer, and the condition of the rifle may have made that worse than it could have been.
Play stupid games, win stupid prizes. I suggest digging out a reloading manual, reading and heeding ALL of the safety warnings and mandatory brass inspections after every firing, and until you understand what you're playing with better, do not deviate from the COMPONENT selections indicated in published load data. I have had a similar accident with a 1911 because I failed to check one important thing after making a slight adjustment to my loading process. Over-confidence is as bad as ignorance.
