I would actually prefer my barrel be sent in a rectangular or triangular box over a tube. Round tubes get beat to death in shipping because the shelving in delivery trucks is designed for square/rectangular boxes, not round tubes, and thus round tubes just roll off them and then all around on the floor of the truck. I couldn't begin to count the number of times I've ordered something with some weight to it that came in a long tube and the end of the tube was either cracked or totally missing and re-taped over. Even had two occasions where USPS delivered empty tubes due to one of the end caps coming off at some point.

As for protecting the ends of the barrel when packaging, it's no different in rectangular or triangular boxes than it would be in a round tube. As in this case, one can leave excess length of bubble wrap extending off the ends to form a cushion area. Another option is to cut hard foam blocks to properly fit the end of the package to act as a bumper. Biggest thing is just making sure that there's sufficient packaging material to prevent the barrel from moving much within the box and that the ends have extra cushioning.