I am finding it really easy to make swaging dies for these things...so for me it is easier to experiment with that. Can change the shape, size, base etc., pretty easy. Any mold close to the right size can be used "as long as the diameter is a little smaller than the final boolit." That way I can powder coat before swaging and the final result doesn't have to be either lubed or powder coated.
The question that remains on this is, what shape and thickness would I want the base to be able to best obturate to form some kind of gas seal for this situation? It would seem that with such a big difference between the bore diameter of the cylinder and the bore of the gun, this thing is gonna lose a lot of gas any way I do it.