I converted a 20 ga to a paradox gun by getting the choke sections rifled and it works great with the correct conical (WR style) 500 grain slugs. I considered screw-in tubes but was concerned they would either screw themselves out, or screw themselves in so tight you couldn't get them out! The trick here is to use pretty aggressive rifling, a slow twist, and to clean out the leading after ever few shots. As far as regulation, I lucked out and didn't even have to refile the sights (well the gun used to be a 25 bore rifle way back before someone bored it out to a 20 ga shotgun).
