It's actually quite suprising how quickly a shot charge will "doughnut" through even a short section of rifled bore. In as short as 10-15yrds the shot pattern becomes a circle with a big hole in the middle that has no shot.
Just a short study of the Paradox indicates they have a relatively large bore, 28 through 12ga, even the 28ga equates to a 56 cal. The velocities of the ammo currently available is in the 1100fps range with full lead solid projectiles. Given a background in Black powder guns that use full lead and lower velocities the rifling configuration is significantly different than that of our modern rifles designed for cup-n-core bullets.
While it's a novel idea to just hog/hone out the inside of a large bore Handi and leave some rifling towards the muzzle, I think the fundamental design of the bore/rifling will be the limiting factor for the custom job you seek,,
If the Handi was truly a simple, productive and reliable platform for a well functioning multi task single bore shotgun/rifle it would be common knowledge and perhaps even offered by the Gun's maker by now.
It may well work, but I'm afraid the resulting actual real world bullet accuracy and/or shot pattern would be dismally lacking in the performance most of us are used too as compared to a stand alone rifle or shotgun.