Re: help do you know what this is
To start with, it's Belgian and, yes, it has been sleeved. Looks like it's been sleeved quite smaller. Roughly, what is the bore diameter?
As for being Civil War, who knows? Could be, as there were a great many European muskets imported by both the North and the South.
Are there any sign of sling swivels? How about a picture of the whole rifle?