Many people believe that an original Paradox gun is merely a shotgun with some rifling at the ends of the barrel. Not so. A cursory inspection of an H&H Paradox gun reveals:
1. Bushed firing pins - as on a double rifle
2. Third fastener - as on a double rifle
3. Extended tang to reinforce the stock at the grip - as on a double rifle
4. Barrels factory regulated using the same method used to regulate a double rifle
As can be guessed, a true Paradox gun is not just a shotgun that has been modified to shoot bullets. It is actually more like a light double rifle that has been modified to shoot shotgun shells. You can trick out all the shotguns you want by adding some form of rifling but all you'll get is some form of slug gun that is far less than a Paradox gun. You can't make a Rolex out of Timex parts.
Considering it's likeness to a double rifle, if you shell out the bucks for a Holland & Holland Paradox gun you get what you pay for.