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Why would the shape help you float? Shape has no effect on buoyancy, only density.
@freemo it's only laterally symmetric, but longitudinally it can still provide stability. Suppose the legs are enough ballast to keep it from rolling - the barrel shape just has to provide enough resistance to pitching to counteract the weight of the head and stop the animal from tipping forward.
Also airplane fuselages are just cylindrical because that's the easiest shape to design a pressure vessel in. It would certainly be convenient to have a rectangular cross section from the perspective of volumetric efficiency, but it'd have to be far stiffer and thus heavier. Planes that place a higher premium on aerodynamic efficiency - fighter jets, aerobatic planes, gliders - don't generally bear much as resemblance to a barrel.
@khird Ahh your talking about forward tipping, I was thinking of rolling... Ok yea that makes more sense, elongation allows for less equal distribution on that axis.
@khird While i do agree that the shape can effect orientation in the water, I would disagree that is due to barrel shape. A true barrel shape is symmetric and thus would not allow for better stability.
Barrel shapes are known to be optimally aerodynamic shapes (see planes, and blimps) and thus would be more suited to making swimming more efficient in that sense I would imagine. That is also the reason, I suspect, a hippo is barrel shaped, for aerodynamics in the water.