Herbert's comment about using shortened 32-gauge shotgun brass to form larger-capacity cases for hunting loads in the Spencer led me to check some measurements.
Starline .56-50 cases measure .638 at the rim and .560 at the base.
An original, fired Spencer rimfire case found at an 1870s battle site measures .648 at the rim and .575 at the base.
Cartridges of the World lists 32 shotshell dimensions as .640 at the rim and .580 at the base. I will be very curious to see what the current Magtech 32-gauge brass measures.
If shortened Magtech cases work in my Armi-Sport Spencer, I think I will have just the brass for BP hunting loads.
A secondary concern is whether the Magtech brass will fit in an RCBS No. 31 shellholder.