Not sure how folks want to fix this with all sorts off reloading suggestions. It is a 375 Flanged Magnum, it therefore uses the rim for proper headspace. The chambers apear to be cut too deep so the rim is loose in the recess, in fact think of it this way chamber is too long, recess for rim is too deep so when the cartridge is in the chamber the whole thing is loose. When the trigger is pulled all sort of nasty things are going on cartridge probably goes forward and then when fired slams back as the rim is not against the breach face(there is play), this causes pressure etc. Which forces the breach open. This also causes the thinning of the brass on first firing and subsequent split in second reloaded firing.
Pull the bullet from an unfired cartridge, de prime and then cut it open length wise and I bet it will be normal. Do the same for once fired and check.
Cartridges that headspace on the shoulder can be manipulated by only neck sizing etc. With rimmed cartridges if the rim recess is too deep you are stuffed.
Whatever you want to do measure check cut open cartridges etc. But please please please stop firing that rifle in the current condition.....
No, no more firing! You are spot on with your analysis IvW. So i got out my calculator and did some first year maths, here is the story:
1. The broken off back piece of the cartridge weighs 7.9grams, or about 150 grains.
2. The diameter of the shell at that point is 12.9mm or 0.51 inch.
3. The force on that back piece by ratio of area is about 1.8 times the force on the back of the 375 bullet.
4. Assuming the pressure to be 47000psi, the force on that piece is 9536 pounds force, or about 4.4 tonnes.
5. The shell stretches and then suddenly parts and accelerates backwards. This is more violent than the bullet heading up the barrel as it experiences smoother acceleration.
6. The back piece now parted from the case slams into the breach face with a blast of escaped gas to add to the force.
7. This whole event happens, if my calcs are correct, in about 6 one hundred thousandths of a second.
the impulse force of hitting the breach face is a function of the head space. No head space, and it is just the 9536 pounds, as in a close fitting breach. The more the head space, the more the impact due to velocity. For example, gently nudge your truck up against a wall until it is touching. Floor the accelerator and all you will get is smoked tyres. Back off a metre however and ram the thing and the wall will fall over.
8. Bottom line, head space is bad news. Head space and a split case is far worse, potentially catastrophic in fact.
