A half round will be a bit shallow and wide for my tastes. I have used a chain saw file before and touched it up with sand paper and a wooden dowel after and that worked ok...
But, if you have any access to a drill press, head to your local home improvement store and get a drum sander bit of the size needed (1/2" -3/4" likely).
You might need some spare paper for it, if you have steel liners.