I am no expert on Paradox guns, but have some experience with 12 & 16 bore doubles and RB loads.

I suggest the left barrel was never intended for anything but shot. To shoot anything from it other than shot, the projectiles will have to be no larger than the choke. This presents problems in the rest of the tube as it will be undersize there. Some form of cupped wad to hold a ball centred would work - I use cloth patching as in muzzleloader shooting, or a gas check cut from a trap shot wad to do this. Both the petals and the fingers are cut off with side cutters. Only the gas check is used. I load the gas check cup underneath the ball so the cup holds the ball centred. You may not be able to shoot a slug as there is no rifling to help keep it nose-on to the intended target. A round ball with cup wad, or undersized ribbed slug might work just fine.

Plastic wads will foul the rifling grooves. With black powder beneath the plastic wad, plastic fouling can be horrific.

I have a crimpe starter that was designed for paper hulls, It is a smooth cone inside and one could be turned on a lathe easily, given 1" to 1 1/8" stock (bar) to start with. The cone is about 60 degree angle. Anneal the brass necks, load, then run them up into the coned die, with case lube on the outside of the neck. You should get a nice crimp using this system - I do.

Check out Beleg's threads concerning loading balls for his shotgun. One such thread is about Russian shotshells..
http://forums.nitroexpress.com/showflat....true#Post146418