this is exactly correct. by fluting the barrel, you're reducing the geometric moment of inertia...which results in larger deflections for the same applied loads.

for two barrels of exactly the same outside diameter, one fluted and one not, the solid barrel will be "more stiff".

as others have said, fluting removes weight, so in theory, you could have a *larger* diameter barrel that weighs the same as a solid barrel. it is possible that the larger fluted barrel would be more stiff, because the moment of inertia is dependent upon the barrel radius.

-matt