Could you rechamber the cylinder?
Possible reasons for the 23 H&R Mag Single Six
1) Ruger likes that cartridge.
2) Maybe the 32-20 has more pressure?
3) The 32-30 is slightly longer than the H&R, maybe it's too long for the Single Six cylinder? So far as I understand, that's why H&R invented the 32Mag, so they could use the same frame as their 38Spl revolver.
If reasons 2 or 3 are right, you're probably stuck with the 32Mag, however, if reason #1 is right, you should be able to get the cylinder rechambered for $50-100 or so by a custom revolversmith (check
www.sixgunner.com for links to customizers)
Anyone know if reasons #2 or #3 hold true, or do we have to write to Ruger to ask?
I heard of a smith that used to build 41Spl (a shortened 41Mag) revolvers on a Single Six frame. I'd imagine having a centerfire single six frame to adapt would make his work much easier and cheaper than trying to move the firing pin from rimfire position.