I suspect that shotgun chokes might be a bit too short to work well as a Paradox with bullets. If it was a barrel with plenty of metal in it, I believe you might get better results by amputating or reaming out the choke, and cutting full-length shallow, slow-twist rifling.
That shouldn't deform birdshot in this age of shot-cup wads, and while centrifugal force would widen the shot pattern, it shouldn't produce a doughnut pattern. The shot are spinning in a smaller diameter spiral as you move inward from the outside, and the ones in the middle aren't spinning at all.
