.... I have no idea why the Taylors/ArmiSport 56-50 spencers use a smaller rim than the originals.
I seem to recall reading somewhere that the new rim dimensions may have been part of a "safety" issue - i.e. so that new ammo (possibly loaded fairly hot with smokeless powder) would not be used in original Spencers. However, that idea doesn't make any sense to me, for two reasons:
- to prevent chambering of new "hot" ammo, you'd need to adopt a
larger rim diameter, not smaller. (All the smaller diameter does is cause extraction problems after the cartridge has been fired ...)
- in any event, there is nothing to prevent someone from loading hot loads in the cut-down 50-70 cases (or any other cases which function well in original firearms) if they are of a mind to ...
Possibly the real reason is something as simple as a mistake ... or perhaps using some existing tooling for some other firearm, as a cost cutting measure.