Would autoloaders favour 18- or 20-inch guns?

Would a viable 18- or 20-inch autoloader move the battleship-gun sweet spot above 16.5 inches?

Not necessarily, because the same technology would make 16.5-inch ammunition faster and easier to handle as well.

A workable autoloader for very large shells implies related machinery could also improve smaller-calibre weapons. Lighter ammunition would compound gains in handling speed, stability, and sustained rate of fire. The original calibre tradeoff therefore shifts less than the headline technology suggests.