Mystery 50 Caliber Cartridge

[rcathey]

Hi all.

Looking for a bit of help on a caliber ID.

I have an old Hopkins and Allen falling block shotgun that someone put a rifle barrel on.
The caliber is not marked in any way.

I picked up some Cerrosafe and this is what I came up with:

Attachment 103535

At first I thought maybe 500 Smith and Wesson but it's too short for even that.

Any thoughts?

Information

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[Sunray]

The 1.545" case length suggests a revolver cartridge. H&A made their falling block from 1885 to 1908 and there was a .45-70 barrel made. They apparently made some falling block rim fire rifles too. Couple of those listed on Rock Island Auctions.
I'm thinking it might be a 19th early 20th Century home made cartridge. The S&W's rim isn't that big. You slug the barrel?

[rcathey]

The barrel on this one is brand new.
The .505" dimension came from a portion of the Cerrosafe that went down into the rifling.

[Parashooter]

Dimensions resemble the U.S. .50-45 Carbine round (based on a short .50-70 Gov't case). A bit long and ~.010" tight at the base, but not far off - especially if the .555" measurement wasn't taken right at the rear end of the chamber.

What's the groove diameter? (Would be about .515"+ for the old .50 cal.) How many grooves?

[rcathey]

That looks like exactly what it is.

I looked at lists of cartridges on Wikipedia earlier and, not surprisingly, that wasn't on the list.

I did manage to find one forum thread (elsewhere) where a fellow had rebarelled one of these into 50-70...so people were doing stuff like this.

Interesting little rifle made more interesting by knowing the caliber. Thanks for the help!