Guys,
We have a couple of posters deciding they know for sure.
Seeing we are going there...I am leaning towards a closed-breech overpressure situation from too much headspace. Too much headspace can be either from a breech that is too fat, and/or cases that have been sized too small.
The fact that only three cases have the ballooned effect on the case body suggests that there was a range of headspace going into that breech. The OP also confirmed a range of headspaces, which suggests a Normal distribution bell-shaped curve. One tail of that distribution curve will have a few cases with generous headspace. This marries with only three cases ballooning. And one had so much headspace that it exploded in the least supported spot at the time. The weakest spot in battery is where there is no lug; where the extractor is.
If it were early unlocking I imagine the extractor claw would have sheared off rather than have the strength to rip that case open. But I am not completely sure here...maybe it is the extractor claw from Hell.
That said, the early unlocking theory also sounds possible.; Slow powder/short gas system = higher pressure on unlocking. A few cases with too much headspace are getting expanded too much and while hot and stressed are then being dragged out with residual pressure expanding them even further as they travel towards the wider breech mouth. One failed in this state.
All that said, I'm not going to lobby for any one possibility at this stage.
We have a couple of posters deciding they know for sure.
- Suggesting it is "text book" adds faux credibility to an assumption.
- Lapua does not have a 'tendency' to do this when it kabooms. Because it does not have a tendency to kaboom.
- Quickload is not "useless" and works for me in this calibre. The pressures/velocities may be off sometimes, but close enough to be useful and no reason to throw the baby out with the bathwater - it is a computer simulation with a lot of nuanced utility. The opportunity cost are manufacturer load tables, the books, and this forum. I don't trust any source and prefer to weigh them all up (I don't have the books).
Seeing we are going there...I am leaning towards a closed-breech overpressure situation from too much headspace. Too much headspace can be either from a breech that is too fat, and/or cases that have been sized too small.
The fact that only three cases have the ballooned effect on the case body suggests that there was a range of headspace going into that breech. The OP also confirmed a range of headspaces, which suggests a Normal distribution bell-shaped curve. One tail of that distribution curve will have a few cases with generous headspace. This marries with only three cases ballooning. And one had so much headspace that it exploded in the least supported spot at the time. The weakest spot in battery is where there is no lug; where the extractor is.
If it were early unlocking I imagine the extractor claw would have sheared off rather than have the strength to rip that case open. But I am not completely sure here...maybe it is the extractor claw from Hell.
That said, the early unlocking theory also sounds possible.; Slow powder/short gas system = higher pressure on unlocking. A few cases with too much headspace are getting expanded too much and while hot and stressed are then being dragged out with residual pressure expanding them even further as they travel towards the wider breech mouth. One failed in this state.
All that said, I'm not going to lobby for any one possibility at this stage.