The 6.5x55 service load was generally around 45k CUP. But all the actions were proofed up to 66k PSI (or the CUP equivalent). To get a catastrophic failure on a Swede action takes about the same level of carelessness as it does on a newer action.
I switched over to neck sizing my 6.5x55 brass as the chambers are "generous" to say the least. Some folks say you can use Lapua brass and full length size every time, but the chamber in my M38 was enough to cause bulging between 1/3 and 1/2 up the body from the case head on the first firing. Can't blame the Winchester brass for bulging into a chamber that oversized.
Machine guns are awesome until you have to carry one.