Oh, well, that will make finding ammo much easier!That strongly suggests a coiled case chamber as they had a separate case head attached to a coiled brass case body similar to a shotgun shell. There was a taper from the base forward and then a mostly straight case to the mouth for the coiled portion. Like the ones on the left in the picture below:
You might try taking diameter measurements of your chamber cast all along the length to determine if it is a drawn-case straight taper or two separate tapers as found on a coiled case.