This is an example of a ball starter and powder measure tool. A soldier would use the wooden part to push the musket ball a little ways down the barrel, then finish pushing it down with their ram rod. The concave piece would be used to measure out a portion of black powder. For source and more details, click here.
Image from: http://www.nps.gov/history/museum/exhibits/revwar/image_gal/vafoimg/frameshell.html