Destroying the barrel would be foolhardy, it's just like pulling a knife out of a deep wound. They don't know anything about the barrel, so it's more wise to take the barrel and examine it at a later, safer time. As to why Shikmaru didn't pick chuunin and Jounin, the reality is that there aren't any to spare. Setting up trap ahead of the sound 4 when they can't even catch up? ..... I refuse to further comment on that.