Guys,
My opinion here, not based on any extensive research. A longer barreled gun should balance as well as a gun with a moderate length barrel. I try to make the barrels so the gun will balance right at the point that the shooter's left hand will naturally hold the gun when offhand shooting. This means when we make a long barrel, like the 56" one above, we must pay close attention to making the muzzle end rather light. I believe that this is one reason for the fowling piece long barrels being octagon to round, to get some of the weight off the muzzle end. The round part of the barrel is a lot more difficult to make than the octagon. It seems to be rather natural to forge an octagon on the anvil face, to make a round we go to a 16-agon then a 32-agon then round. although the longer barrel guns are quite unwieldy in a Pennsylvania laurel thicket, they should balance as well in the hand as a more typical 42" barrel gun.
Jim