buckstix, I don't sleep so well when thinking you feed the old collath with 20 ga plastic shells. here is one method I learn from an 1865 german hunting magazine once.
you have a very long cone between case mouth and rifling and the 20 ga is probably to big inside for the .597 bullet you need. there is a simple way to extend the magtech brass that is even bigger in diameter inside than plastic shells.

take a piece of paper of the case length you want in the end. the paper thickness and the caliber you wish to have in the end definite the width. its try and error until you have it.

coil the paper around a thinner tube

insert the paper roll into the brass case and remove the tube

the paper roll have some tension and will open up until it fits the brass case inside

its ready now, you can load the case as is. I would dip the paper in fluid wax carefully until the brass case mouth. this will also glue the paper to the brass. enough the case will be safe to handle. load with blackpowder, a wad and then glue the bullet also with hot, fluid wax into the case