Why 17-inch guns offered little over 18-inch weapons

Why were 17-inch naval guns not widely developed?

They imposed much of the weight and cost of an 18-inch weapon without delivering the same striking advantage, making the extra increment to 18 inches more attractive.

Capital-ship gunnery emphasised penetration and destructive effect after armour was pierced. A 17-inch system sits too close to the weight and engineering burden of 18 inches to offer an efficient compromise. Moving from 16 or 16.5 inches to 17 inches therefore buys too little, while the final step to 18 inches provides a clearer striking advantage. British and American assessments nevertheless considered intermediate sizes, illustrating that contemporary intelligence and design estimates could still produce awkward calibre choices.