I've never seen one of these, but using my imagination... WAG
I think the spigot idea is it.
The end with the through hole would have the handle/valve part in the tapered vertical hole, tensioned with a washer spring and nut.
The other end with a bottom in its cup would go inside of a galvanized...
What you have is a Chinese cash coin.
I can't read the reverse, but the front reads
Dynasty: Ch'ing (1644 - 1911 a.d.)
Emperor: Sheng Tsu (1661-1722 a.d.)
Reign Title: Kang Hsi (1661-1722 a.d.)
It would seem that you are fairly convinced that this could be a gun part.
Most of us would say it is a piece of pipe.
Do some electrolysis on it and see if it shows any more detail.
Even if not a gun part it still could be civil war era.