IMHO the big Martini or Rem. 'Roller' is too big for a project like this, though if it is what ya got........
The small Martini '.310 Cadet' size or the Mdl. 12 or 12/15 action, which is only 1/4ish inch thicker is the right proportions.
As said, since the H&R comes in .357 that is the thing to use and just download the cases or use that long bullet in 38Spl. cases.
Good point about the 450 Martinis or the #1 Rollers. Better to pick a 'baby' rolling block or a Cadet for such a small cartridge. There are bunches of single shot actions out there and not all of them are in 'collectable' pristine condition. Originals that can be mechanicaly fixed but are too far gone for a real 'resto' or reproductions that have been abused and got rusty or whatever. I keep my eyes peeled and you can find workable actions for reasonable money from time to time. In general, these are meant for black powder cartridges, but if you keep the chamber down the 38 special length and never put a magnum into it you could go smokeless with a good solid example of some of the older actions.
One way to go if you really want a 38 special only rifle would be to turn back the shank on a 357 Uberti Baby Rolling Block. I did it to convert a 9mm Largo Spanish Destroyer to 9x19mm. It's easy. Just remove the barrel, face the shoulder back enough for the barrel to screw into the receiver one more turn and face the chamber accordingly. It just screws in that much shorter. It's important to measure accurately so the sights are back on top when you are done
. Anyway, the chamber is now that much shorter. You may need to touch it up with a hand reamer. You'll also need to file the extractor notch or notches back in. No big deal.
Neither of those options is as cheap or as easy as getting started on loading. That is the best dollars for doughnuts way to solve the problem of feeding a suppressed 357 Handi. I'm just throwing out ideas because you have got me thinking, so I'm sharing my thoughts.
On another note, I don't think that a chamber spacer will do what you want it to in this case. I think it may impart distortion to the bullet before it touches the rifling. Much more so than you'll get by shooting specials through a magnum chamber. Really, the extra chamber length is less of an issue in a break open rifle than it is in a revolver where the chamber and the barrel are two pieces. If there is any accuracy potential loss, I'm unaware of it. I shoot some specials through my magnum and have never noticed anything that made me want to fix anything. I load up mild magnums for it because 1) it's cheaper and 2) Full lenght brass will tend to prevent chamber fouling.
That's another issue, putting a 'forcing cone' in there may create a lead build up point in your barrel.
