A review for the uniniated: The problem with 8G industrial shells... they are high pressure shells used to blast slag out of furnaces. To prevent these shells from chambering & certainly damaging fine old firearms, these industrial shells have an added raised brass ring under the case head. These rings can be altered with a special size die or by turning & cutting so the ring can be pried off.
I am told by those who've used blackpowder in plastic shells in other gauges is the plastic melts. It was suggested to me that it was a poor investment of time to rework plastic shells, though paper shells seemed to reload adequately according to my sources.
Hansome all metal shells can be lathe turned... Austin at Scale Artillerey has promised a batch of 8G cases in your choice of brass or stainless.