In a 6.5x55 swede with a "generous" chamber I found Winchester brass expanded about halfway up the brass body.
Which convinced me to neck size 6.5x55 Swede in milsurp chambers. The brass is just fine as long as it can expand just a little bit and be supported by steel, but a full length resize gave me more than three thousandths difference, which is "excessive" reworking of the body of the cartridge in my opinion.
In rounds that have "cooked off" on the ground, the brass usually bursts at the shoulder. At least in 5.56 blank ammo with a crimp. Evidently the force of opening the crimp is greater than bursting through the shoulder like an overfull balloon.
Machine guns are awesome until you have to carry one.