A specially chambered inside lubed .32 rimfire with a breechblock made by myself, because I have a large supply of primed but never loaded Fiocchi 9mm. shotshell cases which were sent to auction many years ago by the Birmingham proof house. They give light bullet pull with .308 bullets, but should be fine with .312. Then when the cases run out, rechamber to .32-20.
For a modern or high-wall action, a recreation of a vanished and undocumented cartridge of which I have seen only a chamber cast. It can be made by grinding away the bottom of a .300H&H die until a .45-70 case fits perfectly, and whatever and wherever it originated, it looks very much in period for a Winchester Single Shot.