Greetings!
I'm no stranger to case-forming modern brass to work in older arms; this idea came to me shortly after blowing my wad on my 1860 Romano.
I'd had success annealing & fireforming .348 Win brass into .45-75-350 & .50-70 Gvt in the past. So, I took a dozen of those cases, and cut them down to 56-50 length; they were then loaded as usual. The rim is too small for the blade extractor to really get a good hold, but if I held the muzzle up when I cycled the action, the cases would "eject/fall out of the chamber" with reasonable reliability. The magazine to chamber feeding cycle was flawless.
Now, I'm not recommending this for Competition, and this is a time consuming process. But if you can't get proper .50-70 to trim, or 56-50, but do happen to have access to new or once-fired .348 Win brass, you can go out to the range and have fun. Actually, if the new "Lane" extractor in the Armi-Sport Spencer is spring-loaded, these ersatz Spencer Ctgs may even extract/eject a little better.
Again, this is merely a stop-gap expedient, and I did have quite a few .348 Win cases. But if you get handed a couple fistfulls of .348 Win brass {I believe that Winchester still lists it as a component}, and you don't own a Win. 71, give it a trial.
Be Well!
MTM
6-06-05