> Battleships were designed with an "immunity zone" - a range where incoming fire of the designed caliber (generally the same caliber as the main battery) would be at a high enough angle and low enough velocity to fail to penetrate your belt armor, and shallow enough angle to not defeat the relevant deck armor.
Did you accidentally switch belt and deck armor? Isn't deck the one protecting from high angle and low velocity, and belt armor protecting from shallow angles?
I didn't switch it, and you're right - belt armor is most likely to be hit at shallow angles, and deck armor at high angles. That being said, a point-blank shot will still treat belt armor like paper - and a max-range shot will go right through the deck. That's because both the deck armor and belt rely heavily on angling - you get a lot more effectiveness for a given thickness of armor if your opponents are courteous enough to hit it at an angle.
Which means that the belt armor is most effective at max range (though also the least likely to be hit) - shells coming in at 45° will bounce right off. And deck armor is best up close - 5° down angle is going to come up against quite a few feet of metal before managing to penetrate the handful of inches of thickness you could fit in a deck.
The trick to an immunity zone is to find the balance between the two.
Did you accidentally switch belt and deck armor? Isn't deck the one protecting from high angle and low velocity, and belt armor protecting from shallow angles?