That new boolean feature is great! I was messing around with it. Why don't you just use a sphere instead for the barrel. Cut away a sphere like I did in these pictures. Not sure if this is what you were looking for. But maybe this would work for you. After you cut away the sphere move its left over vertices to the back of the barrel and flatten it. Well good luck anyway.
http://www.beteogames.com/barrel1.jpg
http://www.beteogames.com/barrel2.jpg
http://www.beteogames.com/barrel3.jpg