Geno,
I recently had the fortunate opportunity to handle and examine about 35 H&H guns over a five day period. There were guns of all types, some new and some old. From my personal observance of those guns I can say that I did not see bushed firing pins, extended tangs, nor third fasteners on the Royal shotguns present. Also, if you flip through books about H&H guns you won't see those features on shotguns, just double rifles and Paradox guns. Just to make sure I wasn't going mad I took another look at "Holland and Holland, The Royal Gunmaker" to reconfirm this. That is not to say that those features could not be present on other Royal shotguns. I am sure that a bespoke gun could be ordered with all features.
Regarding barrel regulation. Yes, the basic methods are the same for rifle and shotgun barrels. But shotgun barrels are much less critical to regulate to point of aim. A couple of inches here or there means less with centering shot patterns. Much of the work going into regulating shotguns is spent ensuring the inside of the barrels produce an even pattern. Regulating rifle, and Paradox, barrels is much more critical and time consuming.
All that said, it only goes to support what I posted. Which is, that you cannot take a run of the mill SxS shotgun and turn it into a Paradox gun just by adding some rifling. A mass market shotgun is weak at the face, and the action and stock are not made to take the additional recoil that firing a Paradox cartridge creates. A Royal Paradox by H&H is amply reinforced. Other reputable makers used similar reinforcing on their Paradox type guns. Not to mention, all British guns were/are subject to proof. What would happen if you took a Stevens 311 SxS with barrels modified to make a "paradox" and sent it to a British proof house?
You could start with a well made, quality shotgun with a strong action and ample "meat" around the firing pin holes. Then, to do it right, you could drill the action face and install bushings. You should do something to reinforce the stock, at least at the wrist. Then, again to do it properly, you should re-barrel with proper Paradox barrels. If you are a qualified gunsmith and can do it all yourself then maybe you'll have a nice Paradox gun at a bargain price. If you can't do it yourself then you're probably better, and cheaper, off just procuring a proper Paradox.
I had a conversation with Paul Jaeger several years ago about making a .375 O/U on a Ruger shotgun. He had done some conversions like that before and chose the Ruger because of the strength of the action. He said he was no longer converting them because of "problems". I asked what the problems were and he went into a discussion centered around the weaknesses of a shotgun action and problems of overworking it. He mentioned double firing. He mentioned setback. He mentioned safety. A well known USA double rifle manufacturer also gained some early notoriety by converting Ruger shotguns to double rifles. You can sometimes find those early guns for sale online, most often in in .444 Marlin or .375H&H. These days, they refuse to do a shotgun conversion - for good reason.
But bold individuals continue to make cheap conversions using cheap shotguns as the basis. I wish them good luck and hope they continue to keep all their fingers. After all, anything works until it breaks.
