The reason I wanted to know how to target was because when it comes to surface gun battles I believe I am a better shot than my gunners and the Japanese IN and Merchant fleet. However, as it is alway better to aviod any damage I wish to ensure that I am hitting the enemy as soon as possible, thus some long range shots before his gunners zero in on me. This is easy with the main deck gun as you can zoom in on targets through the aiming scope. However, with the 40mm cannons the targeting rings obscure the view so it is hard to know where your shots are striking.
As for AA guns being ineffective, as I said earlier, the 40mm cannons are IMHO more effective against merchants than the main deck gun. Try straffing the decks of any merchant with a 40mm and see the effect - explosions everywhere and a very quick "enemy destroyed message" This may not reflect RL but that is how the game runs.
