The rupture was mid case along the long axis.
Yeah he nailed it.
But there is a chance you did not measure the case before and again after firing. We do not know what effect the chamber had on the case when fired and then there is the diameter of the barrel when compared to the diameter of the bullet, I want my bullets to create enough pressure to seal the chamber between the chamber and case.
If your powder is too slow or the pressure is recused because the bullet is too small in diameter gas can get trapped between the case and chamber. When the pressure inside of the case falls the outside pressure can rupture the case. It is possible for the case to rupture and the case can collapse inward.
The 45/70 case has a tapper of about .020" from the case head in front of the rim to the mouth of the case. If the chamber is too large in diameter I suggest you back the die out to reduce the dies ability to size the case.
I do not know what a good number would be but most reloaders use fractional turns of the die; I don', I go straight for it. I use a feeler gage to adjust the die off of the shell holder, I like to start with .010".
There were times I went for the magic .002", and then there is my M1917, I go straight for .014" when sizing 280 Remington cases for that 30/06 chamber.
I do not know how other reloaders do it but it seems the fiddle with it until they get there.
F. Guffey