Hello:
Been down that road.
Some tips, please:
The Spencer's, esp. ARMISPORT IMPORTS, have varying chamber dim's.
The metric chamber drawings of the EU " authority" rarely match the chamber.
Plus, the 56-50 rounds are battered, bullets can loosen and have tipped noses from cycling.
Because of cycling bumps, a round that finger chambers may not cycle.
Might be better to provide an uncycled dummy round (of one that cycles) instead of one matching a drawing.
Smiles.
El Supremo/Kevin Tinny