Well, folks, another "piece of the puzzle" appears to have fallen into place in my hope for a practical centerfire conversion setup for my .50-60 Peabody!
One of the local black powder fellows, whom I've known for decades, has a .50-70 Rolling Block ... I asked him if he could spare one or two old cartridge cases so I could explore how suitable they'd be for conversion to ".50-60 Centerfire". He was most obliging and last night dropped off two cases (one sized and one unsized.)
Although some of the specifications quoted for the .50-70 had been causing me some concern - in particular a rather larger rim diameter than stated for the .50-60 Peabody - I hoped that the differences would not pose insurmountable problems. For example, I assumed that the rim diameter could be reduced, if necessary ....
First off, here's a very helpful image provided by fellow-CGN "Beater", who has an original .56-50 Peabody cartridge among his treasures:
As you'd expect, I immediately found that the full-length .50-70 case (1.75 in.) wouldn't quite "make the turn" into the Peabody chamber, though a test with the rim gave promise that it might fit the rim cutout without any reduction in diameter. Other sources available to me give even shorter case lengths than Beater's measurement in the above photo - Barnes: 1.456"; Logan: 1 7/16" (1.4375"). Today I trimmed the sized .50-70 case to 'Peabody length' - settling on 1.475" as a good starting point (on the theory that it would be a lot easier to take a bit more off the length, if necessary, than try to add it back on!

Actually, final dressing of the case mouth left me with a finished length of 1.470" ....)
I am
very pleased to report that the trimmed case chambered like a charm - no problem at all with the rim diameter being too great for the recess, and the block closed up on it very nicely, with that ever-so-slight nudge on the base of the case which I have come to view as a sign of good case fit in a Martini action ...
Here's a quick 'scanner plonk' of my prototype ".50-60 Peabody Centerfire" cartridge case ....
Now for some other observations I find most interesting:
- although the rim diameter of this particular .50-70 case is actually 2 or 3 thousandths
over the 0.660" cited by Barnes, it fully enters the rim recess of the chamber on my rifle, as already stated. (I'm wondering if a rimfire firearm might even normally be a bit "oversize" in this respect, to facilitate complete "squashing" of the rim by the striker, in the interests of positive ignition ...

?) At any rate, I expect that this situation will simply contribute to positive extraction.
- other critical measurements from the .50-70 case are "spot on" in comparison to the measurements in the above photo - diameter of the base just ahead of the rim is 0.562", and the mouth - though slightly out of round - averages 0.5425"! (Low of 0.539", high of 0.546")
- you'll note that the "major" neck diameter of the original round is 0.543", though the crimp reduces it quite sharply to 0.509". Interestingly, for the .50-60 Peabody, Barnes gives a neck diameter of 0.508" and a bullet diameter of 0.499" ... Logan gives no bullet diameter, but a neck diameter of 0.535". (Quite a variance, but may depend on the particualr make of cartridge they were measuring, or even exactly where they took the measurement.)
- Intriguingly, I know that "back in the day" the Canadian Militia issued
.56-50 Spencer cartridges to militia armed with Peabody rifles. Cartridge dimensions are very close, though the Spencer cartridge is quite a bit shorter - e.g. Barnes gives a case length of 1.156" and 1.632" for OAL of the complete cartridge. But this would be very much akin to being able to shoot a .22 Short or Long in a .22 Long Rifle chamber, or a .38 Special in a .357 Magnum chamber.
- On that note, Barnes gives the case-mouth diameter of the .56-50 as 0.543" -
note that correlation.

The
bullet diameter of the .56-50 is given as 0.512" - arguably quite a bit too large, one might think, except that I have slugged my Peabody barrel at about
0.512" groove to groove! (I can only assume that the Peabody cartridge was loaded with a hollowbase bullet, or at least a very soft one, for expansion ... What's more, moulds are now available for bullets of that size, intended for the reproduction Spencers now on the market.)
All in all, things certainly do seem promising! 