If you are using the sheet metal template, there should be a command for 'flat pattern.' That is the only way to 'unroll' a cylindrical shape.
Unfortunately, I don't know if there is a way to create a cone in sheet metal and get a flat pattern. I know it'll do any other shape. I've created some pretty trick brackets and they were flat patterned perfectly. So I know the feature works, but I just don't know if it works for cones.
Maybe someone on here can elighten both of us.