That looks like part of a powder flask.Two pieces like you have there fit together to hold powder,and a brass dispenser at the top measured a charge of powder when a button or lever was pressed.Google search(images) for powder flask to see many examples.They are COOL!
I agree, half od a powder flask. I found the dispensing end of one a week or two ago. Neat! Maybe between the 2 of us, we'll have enough to build one, lol!