Marcus Aurelius wrote: I am not saying that mixed secondaries were a good idea, but in the 1930s only the US Navy and to somewhat lesser degree the Royal Navy and the French Navy correctly estimated the relative threats presented by aircraft and destroyers. The US Navy ended up sacrificing some anti-destroyers firepower for better anti-aircraft firepower, which was ultimately the right decisions. The Royal Navy and the French Navy wanted to have the best of both worlds. The 5.25" DP was a only partially successful until the improved RP10 mountings for the Anson and Vanguard. The French 152 mm DP gun on the other hand were almost a complete failure for AA work and in fact the French had to replace some of them with 100 mm guns. Even after the war the French were unable to fix them completely. So it's not like uniform DP secondary guns were not attempted but only the US 5"/38 really hit the spot.
There's another aspect to this as well. During the era when secondary batteries were regarded as a defense against torpedo craft, the caliber of secondaries used for that purpose was related directly to the size of those targets. The guns were selected for their ability to do a lot of damage to said craft quickly, before the latter could fire their torpedoes. In the early days (up to Dreadnought) the 12 pounder and its equivalents were considered perfectly adequate for this role. This is disguised because the pre-Dreadnoughts carried heavy "secondary" batteries of 6 inch and 9.2 inch guns or their equivalents. These were not anti-destroyer guns but were intended to be part of the ship's anti-capital ship armament, providing smothering fire with HE while the big guns provided penetrating and crushing power. That was the theory anyway. I put the secondary in quotation marks because those batteries of smaller guns were often regarded as the ship's primary armament and the quartette or so of big guns were secondary to them. The "anti-destroyer" (actually anti-torpedo-boat) guns were the 12 pounders and 3 inch weapons and their equivalents.
Now, several things happened in the early years of the 20th century. One was that battle ranges went up and experience showed that the heavy batteries of 6 inch and 9.2 inch class weapons really weren't worth the weight they absorbed. In fact, they were a liability because they meant that magazines for them were dispersed all over the ship. So battleships simplified their armament by concentrating on heavy anti-ship guns and anti-torpedo boat guns. So, we have Dreadnought with 12 inch and 12 pounders. The US did the same with its first class of Dreadnoughts that carried 12 inch and 3 inch guns. The French, Germans and Japanese kept the 6 inch and their equivalents in the ship layout but these were not anti-torpedo boat guns (note that the same ships also carried the 12 pounder equivalents for that role). Those 5.9 inch, 5.5 inch et al weapons were still part of the main battery and were intended for anti-capital ship work. The inclusion of those guns was a highly retrograde feature that showed the Germans, Japanese and French had completely missed the point. The Germans and French could be forgiven, in naval terms they were bungling amateurs. The Japanese had no such excuse; it was their great naval victory off Tsushima that had highlighted the uselessness of the medium-caliber batteries.
Another thing that happened was that torpedo boats started getting bigger. From 1905 onwards this process was fast as torpedo-boat destroyers were evolved to sink torpedo boats and then took over the latter's role. They weren't just getting bigger, they were getting faster and the torpedoes they carried were also getting larger, faster and longer-ranged. It was very quickly apparent that the 12 pounder, 3 inch and 3.4 inch guns used for anti-torpedo boat work were outclassed and inadequate. The requirement was changing; the new destroyers could fire from further out and make their attacks with less warning. This decreased the window of opportunity to sink said destroyers and that needed guns with longer range and heavier shells. Have a look at a 1914 JFS and the surge in size, speed and fighting power of torpedo boats as they evolved into destroyers is very obvious. So, the British went from 12 pounder to 4 inch, the US went to 5 inch. Note that on these ships, there remained only two types of gun, the heavy anti-capital ship guns and the light anti-destroyer guns. There was no secondary armament per se. Once again, the Germans managed to miss the point completely; they held to the intermediate-caliber guns and left the anti-destroyer task to the inadequate 3.4 inch gun. The Japanese and French wised up and they eliminated their light anti-destroyer guns completely. They introduced new caliber guns (5.5 inch) for the secondary battery and use dthem for anti-destroyer work.
By 1912, even the four inch and its equivalents were becoming regarded as ineffective. There was a lot of pressure to make the next jump up to six-inch weapons. The French and Japanese weren't worried; they were ahead of the curve with their 5.5s anyway. The Americans were quite happy with their 5 inch gun and the Germans were happy with their 3.4 inch. In Britain, there was a big debate over the anti-destroyer battery with one side claiming the six-inch was essential due to its range and weight of shell while the other claimed that the four inch with its high rate of fire was the better choice. The six-inch crew won that battle temporarily although they lost it later and the UK reverted to the four inch, then changed its mind again and went back to the six inch as destroyers continued to grow.
OK, now we can jump forward and we can look at the situation in the 1930s. Destroyers had grown a lot bigger, a lot faster and a lot better armed. Eight or even ten torpedoes were standard, guns were 4.7 inch or 5 inch while their speed had jumped from the high twenties to mid-thirties. Even the six-inch gun wasn't going to cut it as an anti-torpedo defense. So what were navies to do? The obvious "answer" to the big new destroyers was a heavier anti-destroyer gun. Was that going to be eight inch? Or 9.2 inch? Or 12 inch? Once again, the obvious answer was the wrong one; the fact was that no battleship of reasonable dimensions could carry an effective anti-destroyer battery. In fact, an effective anti-destroyer gun for battleships couldn't be designed at all. A gun that fired a shell heavy enough to cripple a destroyer quickly couldn't be fired fast enough to guarantee scoring a hit in the time available.
So, the anti-destroyer mission had more or less gone away. That's why "dual purpose" guns were adopted by the British and U.S.; they realized that with the anti-capital ship and anti-destroyer functions both gone, there was no point in looking for anti-surface capability. On the other hand, aircraft were growing threats and that was the new primary role for the secondary battery. Again, there was a fight over that (these things never happen quickly or cleanly) but the upshoot of the debate was the US 5 inch L38 and the British 5.25 L50. In fact, the row continued well after that point, the obvious results being the 4.5 inch secondary batteries on the modernized battleships and battlecruisers. The Germans (of course) completely missed the point and retained the 5.9 inch as a classical anti-capital ship secondary battery while replaced the old 3.4 inch with a 4.1 inch gun for anti-torpedocraft work - except that anti-torpedo craft role now included anti-aircraft.
So, the argument over whether Bismarck et al should have had dual-purpose "secondary" armaments or not misses the point. As the British and Americans defined things, she had a modern dual-purpose secondary battery, her 4.1 inch guns. The catch was that she had a traditional secondary battery as well - and therein lay the problem. A British or American design team in the German's shoes would almost certainly have thrown that battery of 5.9 inch guns away and installed additional 4.1 inch guns. After all, the 4.1 inch fired faster, was easier and faster to swing and train, wasn't weighed down with an armored turret and, to a 1941 destroyer, the difference between getting him by a 4.1 inch shell and a 5.9 inch shell wasn't really consequential. In either case, she'd live long enough to get her torpedoes off.
Imagine Bismarck with no 5.9s but 16 twin 4.1s instead of eight. She would have had a lot better AA firepower - and that might have saved her - and her anti-destroyer capability wouldn't have been any worse. What she would have lost was her anti-capital ship secondary battery - and that had been obsolete since 1905.
PS It should be noted that the above refers to why design choices were made; how the guns actually got used is entirely another matter.
