Factory ammo but that case only got less than a full charge of slow burning powder when the charge was dropped. Remember powder in factory ammo is dispensed not weighted. I strongly suspect S.E.E. Some years back there was an article in Handloader Magazine where the writer was able to repeat the detonations.
http://www.africanxmag.com/secondary_explosion_effect.htm#:~:text=Secondary%20Explosion%20Effect%2C%20or%20S.E.E.,burn%20at%20the%20prescribed%20rate.&text=The%20most%20interesting%20call%20was%20from%20a%20powder%20company%20ballistics%20laboratory%20technician.
"Secondary Explosion Effect, or S.E.E. seems to occur when a "slow" powder is used and the powder charge reduced to below 15% of max. This will cause the powder to detonate rather that burn at the prescribed rate."
That was what came to my mind as well. IIRC, two cartridges were notorious for the S.E.E. problem, the .243 and the 7MM Rem. Mag. in the fact that there were more instances of S.E.E occurring with those two than any other cartridges.
Paul B.
