Patrick,
Many BPCRs (and all those I shoot) have little to no effective shoulder.
The 45-70 is one that I have fired thousands of rounds through using relatively few cases. I have never had a case head separate on a BPCR. I have had them burn through with a pin hole and have had the necks split.
BTW the bullets are always lubed with lots of gooey lube and at times it gets all over the cases and chambers with no ill affects using smokeless powders.