Unknown (to me) Rimfire Cartridge (Union Metallic Cartridge Company?)

[Eastender]

Today in the woods I found this rimfire cartridge marked with a simple U on the base. It may kind of look like it, but there isn't a dot within the U signifying Remington. I'm wondering if it is the pre-Remington merger Union Cartridge Company (1867-1912) in Bridgeport, CT.

My caliper batteries are out, but it looks to be 14mm (.55) outside diameter by 30mm (1.18 or 1 3/16") long. Any guesses for the round? Thanks

 

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[Eastender]

.56-.56 Spencer

 

[Gare]

i would go with the Spencer also Sure looks like one

 

[diggerdeeper]

The firing pin indent definitely looks like a Spencer or some early cartridge conversion that chambered the Spencer round.
Winchester RF rifles used rounded pins so left a characteristic "center punch" mark.

 

[Digger RJ]

Today in the woods I found this rimfire cartridge marked with a simple U on the base. It may kind of look like it, but there isn't a dot within the U signifying Remington. I'm wondering if it is the pre-Remington merger Union Cartridge Company (1867-1912) in Bridgeport, CT.

My caliper batteries are out, but it looks to be 14mm (.55) outside diameter by 30mm (1.18 or 1 3/16") long. Any guesses for the round? Thanks

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