I too am no expert but I do shoot 12 bore Paradox guns.

I find that shooting shot cartridges loaded with fiber wads produces the best results in my Paradox. With fiber wads I get patterns very similar to those produced by a light-modified choke. Fiber wads also leave a cleaner barrel.

What Daryl said about your smooth bore barrel, "never intended for anything but shot", makes good sense to me.

If you load shotshells in both barrels then you have a versatile shotgun, i.e. open choke by tight choke. Load a shotshell in the left barrel and ball or Paradox cartridge in the right and you are using your cape gun as intended for mixed game. Regardless, I suggest fiber wads for both barrels.

~