Sorry for the very late (+1yr .. cough) reply. Not on here much these days. (Most chat seems to be on the Wild West Airsoft chat group on FB.)
The Cassiopeia shells are discharged by an impact at the front of the shell, not the rear. In the revolvers, this is done with a lever and a circular tube that when fired, pushes back into the front of the shell against a circular seal.
So i made my own barrel/hop assembly that the shells rest against. The falling block is skimmed to make a little thinner and is held off the rear of the shell by a spring. When fired, the hammer falls, pushes t