I would do a cylinder and cut in an edge loop. Then scale that loop out. Then cut the other two loops and scale those out. Now you have the rounded barrel. Last I would split those loops and extrude outward to create the metal rings. Simple as that.
EDIT: Added an image to help. In the wise words of LeonLabyk "Nice Barrel."
Last edited by Chavfister; 31-12-2013 at 12:53 AM.