6" difference in barrel length . . . it'd be pretty hard, although not impossible, for tubes with that much difference (fit to the same gun) to weigh the same. It'd take a lot more barrel wall thickness on the short ones, for sure!

On well-made guns, if there's not a lot of difference in length, two sets of barrels can be essentially identical in weight. I have a 12ga Parker Repro 2-barrel set, 26" and 28". On my digital postal scale, there's half an ounce difference between the two. Would be interesting to put that gun on Don Amos' machine and take readings.