... Longer might have an edge for crossing shots, but if both guns are balanced at the same point what's the advantage of one over the other?
I think the difference (some call it an advantage) is that longer barrels, even if balanced exactly the same as shorter, will have a different moment of inertia and will therefore both acquire and shed momentum more slowly than shorter barrels. That is, the longer barrels will take longer to get moving and will keep moving in a swing when shorter barrels could and would stop. Balance fore and aft is one thing, momentum in swinging the gun, another.
