I found this

[Utah Bob #35998]

While exploring an old abandoned mill in NM. Is it a 45-70 case?

[Yul Lose]

Looks like a rim fire case, no primer pocket.

[Michigan Slim]

Looks like one of the old copper cases that separated and made the gun a club.

[Rip Snorter]

Likely this

BENET-PRIMED - A common style of inside-primed car-tridge developed by Col. S.V Benet commander of Frankford Arsenal in the late 1860s. It was used extensively in early U.S. military ammunition. A copper or iron cup was secured inside the head of the case by characteristic crimps. This cup served as an anvil to enable the firing pin to activate the priming mixture and also to reinforce the head of the case.

[J-BAR #18287]

Totally guessing here; maybe .56-56 Spencer?

 

Wiki:

 

[Dantankerous]

Whatever caliber, a very neat find!

[Abilene Slim SASS 81783]

Based on its proportions, it looks to me like the .58 Springfield shown in this chart. 

[Smuteye John SASS#24774]

2 hours ago, Yul Lose said:

Looks like a rim fire case, no primer pocket.

The Benet priming system has the primer inside the case like that.

[Alpo]

Whatever it is, it appears to have been a lost cartridge, as opposed to a used piece of brass, since I see no evidence that it's been fired.

[Pat Riot]

Neat find. It’s amazing that its not corroded. 
 

Just guessing here but looking at Slim’s picture could it be #9 the .50-45-400 Cadet?

[J-BAR #18287]

I bet it would have been loud in that mine!

[Utah Bob #35998]

7 hours ago, Rip Snorter said:

Likely this

BENET-PRIMED - A common style of inside-primed car-tridge developed by Col. S.V Benet commander of Frankford Arsenal in the late 1860s. It was used extensively in early U.S. military ammunition. A copper or iron cup was secured inside the head of the case by characteristic crimps. This cup served as an anvil to enable the firing pin to activate the priming mixture and also to reinforce the head of the case.

Bingo. I can see the two holes at the base on the inside!

1 hour ago, J-BAR #18287 said:

I bet it would have been loud in that mine!

A mill, not a mine

[Utah Bob #35998]

7 hours ago, J-BAR #18287 said:

Totally guessing here; maybe .56-56 Spencer?

 

Wiki:

 

Looks too long for that. And no crimp.

[Utah Bob #35998]

6 hours ago, Smuteye John SASS#24774 said:

The Benet priming system has the primer inside the case like that.

One side of the case is very dark after cleaning. It could have been in a fire and detonated. That would explain the lack of any mark from a hammer strike.

[Buckshot Bear]

Cool find there Bob.

[Charlie Harley, #14153]

Looks like a good find after a fun day exploring.  Happy for you, UB!