A few years ago I sold some 50/70 rimfire cartridges to a dealer
and he paid me $50.00 a round.  It is quite rare and collectable.
If you wanted a round for a display I could give you the dealers
phone no.  I have never seen any since for sale.  I probably 
sold it too cheap.
 
Charlie

If y trying to shoot something, I have read that some have made the Dixie Gun Works 56 spencer ( I think it is called ) reloadable rime fire work in a .50. I am not sure if the case would need turned down a little or not. Might be worth a look. I have one of the cases I could measure. They are short for sure. But I don't kow if the base/neck would fit up or if it needs much work to make it do so. I bought one to have on hand "just in case"!

Merwin

It is possible to make rounds for large caliber rim fire rifles using a .22 (dismantled) as a primer.

I have made Joslyn, 54 Ballard, Sharps & Hankins, and .41 Frank Wesson cartridges this way.

All were made out of brass tubing with a base silver soldered into the tubing.   A .22 cal hole was ofset drilled into the base so that the rim of
the .22 was on the "rim" of the desired cartridge.  The hole dia has to be drilled so you have to tap the .22 into the case.  Then the hole has to be spot faced so the rim of the .22 is flush with the case.

You have to pull the .22 bullet and dump the powder.  It's worth while to prime the .22 case with fffg or even ffffg to insure ignition.    Fill the case with black powder, thumb press the proper bullet and it is ready to fire.   

On loading, you have to orient the cartridge so the .22 is placed where the hammer can hit it.

I tend to think that I was one of the few people in the US to have shot the .54 Ballard Rimfire Carbine which was surprisingly accurate.

It's quite a job to make a handful of these cartridges.   You need a metal lathe, and at least a drill press. A mill is better as you need to be precise to drill the hole so the .22 rim is in the proper position.  Both the press or mill will need a wiggler to get you into the proper position.

These cartridges can be made by any machinist or gunsmith.

Dixie Gun works sells similar... (You need to Login or Register to view media files and links)

Merle

Dixie used to sell adapter cases for several rimfire calibers that used .22 blanks to ignite them. They were lathe turned with a .22 cal hole on one edge to fit the blank into. You just inserted a .22 blank, filled it with blackpowder, and set a ball on top, then chamber it so the rimfire case lined up with the firing pin.
I had some for .44 rimfire and they worked well. Only problem was knocking the spent case out, as they open up slightly larger than the hole, so they had to be tapped out with a punch.

If you can obtain a centerfire breechblock, perhaps from the current makers of the Peabody, you can have cases made by Rock Mountain, assuming that nothing more common can be adapted.  Greatly simplifies the equation!

David