A piece of threaded pipe the same diameter of the barrel works great for finishing up the rounded portion of the barrel channel. Just drag the treaded portion along the bottom of the channel. This will really do a good job of seating the barrel all the way down. If you are feeling really froggy you can cut the treaded portion off the pipe and fit a dowel into it and make something like a plane. Just make sure that your diameters match and go for it.
Jack