Need help with a "Paradox"AR15 in .223

Hello everyone, first post here.

So basically I live in country where automatic & semi automatic rifles are prohibited for normal citizens. Well technically you can get those firearms if you apply for manufacturer or other higher forms of license. But it takes time, somewhere between 1 to 7 years just to be exact.

I have never fired a semi or full auto rifle & don't even know if I would even like those to begin & hence I don't want to dedicate so much time on achieving something that I may not end up even liking. But here you can buy a smooth bore paradox gun, which basically means its a "shotgun" with a rifled choke. I was thinking of experimenting with a .223 with a 20 inch barrel part of which is going to be smooth bore & part rifled.

I would greatly appreciate if someone could help me with the rifling twist rate, length of barrel which needs to be rifled & most importantly how different it would be (ballistic wise) compared to a fully rifled 20 inch .223 gun.

Some laws you may want to know.
1) Barrel cannot be shorter than 20 inches.
2) The rifled part needs to be a screw in choke.
3) The rifled part cannot be greater than 50% of the barrel.