Out of curiosity and without a full understanding of the NFA ruling...is the only reason shotshells fired from pistols are legal is because the barrels are rifled? I have built shotshells for .44 Spl/Magnum pistols and .357/38 special pistols for years from shotshells caps provided by Speer and loaded with 7.5 shot.
By the way, how do you get a 2.5" 410 shell into a 1.5" 1917 cylinder? I would have to agree that modifying a 1917 to work like a Judge firing .410 shotshells out of a smoothbore barrel is just looking for trouble.