Saw that! Pretty cool. Alan only did that on an old model.
Enlarging the loading trough enough, skeletonized the trough so much that on a new model, it cut into the transfer bar milled channel and exposed the bar so that it interfered with loading rounds. But that just adds to the coolness factor and challenge to make it work.
I have a .32 H&R Mag new model of course, and made convertible 327 FM and 32-20 'Short' cylinders for it that were much simpler. The 32-20 was a bit of a challenge but not much. Enlarging the loading trough for the larger rim, .015" more than the same size as 9mm (but .031" smaller than the 38 Spl) was nothing and did not skeletonize the trough. The cartridge length was not that big of a problem either for a reloader. I just deep seat the bullets like the old S&W 32 and 38 target ammo with bullet completely in the case, although I don't have to seat them that far. Or I just trim the cases 1/10" and seat the bullet to normal depth.
A 9mm at 35,000 and 9mm +P at 38,500 ANSI/SAAMI doesn't seem to be much of an issue in a standard size single six 5 shot cylinder. An uneven # of chambers of course moves the cyl notch (the weakest link) to between the chambers. Considering the 327 Fed Mag is 45,000 ANSI/SAAMI.
But that's not the whole story; while the 327 chamber wall thickness is .070", the chamber wall for a 9mm chamber is reduced to .044". Is that a game changer with the 9mm's lower chamber pressure? I don't know.
Anyone have a Single Seven that can give us the chamber wall thickness between 327 chambers?
But even making a single six cylinder .052" larger in diameter to obtain a .070" chamber wall (removing .026" from the top strap and bottom of the cyl window) seems like a minimal factory alteration. But again, there's retooling costs.
And that would easily put the 38 Spl even 38 Spl +P at only 18,500 on the table for a convertible.