Hello, Ernest:
There are slight differences in the chamber and case dimensions between original Spencers and Armisport 56-50's. Also, Armisport 56-50 chambers varied among themselves.
I believe you would be better off with "Starline 56-50" cases, that were designed for the Armisport Spencer by Ken Howell.
I have been waiting two years for Starline to produce this case. Hunter Pilant, Production Manager, can be reached at:
Hunter@starlinebrass.com.
Alternatively, Rocky Mountain Cartridge in Worland WY can make them on a custom basis. Kody at RMC, 307-347-4547, can answer questions. If use RMC, tell Kody you have an Armisport, and be especially sure of your gun's chamber and case rim diameter, along with requiring case mouth wall of .015", and a case mouth wall just above the web of .025".
If you are mechanically inclined, 50-70/90 Sharps cases can be shortened, annealed and rim narrowed to fit. But these cases are also scarce now.
I can supply you with some Starline 50-90 Sharps cases, and details about to rework them.
Oh, there are old Forum threads/posts about using 348 Winchester and 8MM Lebel parent cases to form 56-50's. Skip this headache.
Smiles,
Kevin Tinny
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