I would argue that the "fleet destroyer" didn't stop being a "fleet destroyer" during WW II simply because the gunline lost it's former supremacy in fleet tactics. It was still necessary to protect the fleet against a number of threats and perform several new functions (such as plane guard duty) when the aircraft carrier came to be the primary capitol ship. The fleet destroyer simply evolved from one armed with surface guns and torpedoes to to one armed with DP guns, torpedoes, and ASW sensors and weapons such as depth charges and hedgehog. So no, the fleet destroyer didn't become superfluous, as you claim, it evolved to meet changing threats and requirements. In fact, the "true" fleet destroyer was no longer one armed with only torpedoes and surface guns as that kind of ship could no longer adequately protect the fleet; it became the multi-role destroyer.
Basically I agree with you that the role of the destroyer changed...nevertheless it is no longer strictly a "fleet destroer, is it.....
In fact, the 5"/38 had few practical disadvantages. lets dispose of the range issue; You admit that the range of most destroyer engagements in WW II was 12,000 meters (about 13,200 yards) or so. At that range, the 5"/38 is in it's element. The gun elevation of which you make such an issue is about 19 degrees. BTW, At 17,000 yards, the 5"/38 gun elevation (with the Mark 46 round) is 35 degrees not 45 degrees, as you claim. Incidentally, I'm not limiting the claims made for the 5"/38 to strictly destroyer armament; it was used very successfully as secondary armament on both battleships and cruisers.
Your assertion about it being 35 degrees is only true for less than 10% of the usual ammunition load out of a USN DD. The overwhelming majority of the ammunition did not have the characteristics you describe. Both the AAC and the "Common" with solid fuse cap required elevations in excess of 45 degrees to achieve the ranges of 18000 yards
Whilst we are "disposing" of superfluous issues, we might as well get rid of the rate of fire issue. In theory the 5/38 had a practical rate of fire of 25 rounds per minute, however, with a total ammunition availabiliuty of about 150 rounds, this could not be sustained in a typical surface (or air) engagement.
Reference to battleshiop and cruiser fitouts has no relevance to this thread. The same arguments could be mounted to the german 5.9 guns (or any guns really) which were very successful in cruiser deployments
Please don't try to put words in my mouth in order to make your arguments seem more reasonable. I did not say US destroyers were able to "effectively engage out to 17000 yards and beyond," What I said was that Crenshaw, a former destroyer gunnery officer, mentioned that US destroyers in the South Pacific could, and did, open fire at 17,000 yards and frequently made hits at that range. Crenshaw does not give details of these hits and mentions the feat only in passing. I believe there were hits at similar ranges by 5"/38 batteries, but possibly not those mounted on destroyers.
I dont need to put words into your mouth, you are doing that quite well by yourself. Effectively engage, means being able to register hits in my opinion, and here you are, saying that " hits are ferequently being made at that range" If that is not arguing that the 5/38 was effective at that range i dont know what is.... If you are going to claim hits at that range, in action, please give some examples and we will have a look at them. i know of none, and i suspect neither do you.
A difference in trajectory of 5 degrees does not translate into a much more effective gun, especially when you consider a whole host of problems with which the Japanese 5"/50 was plagued, and which you conveniently fail to mention. ......... these problems rendered the Japanese gun far less accurate than the 5"/38-Mark 37 FC system. So not only were 50 % fewer Japanese shells being fired in any given period, those that were put in the air had far less chance of actually impacting a target.
Would be true except the trajectory is 45 degres at 18000 yards (and you did say 17K +). Moreover, whilst the 5/38 could fire at that rate, it could not sustain such a rate, due to the limited supplies of ammunition carried.
The Japanese demonstrated their adequate gunnery skills time and again in Pacific battles. The gunnery was sufficient to keep the American destroyers at bay, whilst the Long Lances went to work. Or are you going to try and argue that the Japanese were not more effective than the USN DDs during surface engagements (until the very end of the war).
Furthermore, the slew rate of the Japanese 5"/50 was very slow compared to the 5"/38, which along with the pathetic ROF of the Japanese gun, not much better than the US 8"/55 heavy cruiser gun, meant Japanese destroyers were at a marked disadvantage in gunfights with US destroyers. If it had not been for their superior torpedoes, they would have been slaughtered in the South Pacific.
Slew Rate was 6 deg/sec. i know of no competent technical assessment that finds either the rate of fire, or the slew rate being too slow in a surface battle. The effective ROF is variously given as 6-12 rounds per minute, depending on the details of the turret type.
At 12000 yards, Jap turrets are able to traverse 628 yards/sec of distance. Your DDs need to be travelling at over 1100 knots to outpace the Japanese turret, at that range. if the distance is halved , to 6000 yds, the turret is tracking at 314 yards per second, still the target needs to exceed 600 knots to get away from the turret. the japanese had no difficulty in tracking surface targets with these turrets, and i again know of no balanced assessment that supports you on this issue.
With regard to your observation about the japanese being at a marked disadvantage against US DDs in a gunfight, I have to disagree with that opinion, and rely on the comments made by Campbell ..." During WWII there were instances of very good and very bad gunnery Japanese gunnery against surface targets. Such variations occur in all navies and Japanese shooting was on the whole similar to British and American..." Campbell also gives a description of the Japanese Surface Fire Control, and nowhere does he say that the system was notably deficient.
I am not saying thet the 5/38 was not much faster, or that the FC system in the US ships were not better, although I dont agree that the japanse ROFs were "not much better than US Heavy cruisers, in fact they were 2-3 times as fast....
Which, of course, was almost totally irrelevant. The 5"/38, as were almost all destroyer guns, was designed to counter other destroyers, not armored ships. The armor penetration of the US 5" shell was more than adequate for it's intended role. Your calculation that the Japanese Type 3 shell was 73% more effective against armor is pretty meaningless since Japanese destroyers seldom engaged US armored ships with gunfire. As for ammo loadouts, these varied during WW II and commonly reflected the anticipated needs of a destroyer for any given period. It doesn't make much sense to fill a destroyer's magazines with armor piercing shells when the IJN will be attacking the US fleet mainly with aircraft.
Ah, the loadout could vary, but the the two most common, the AAC, and what I refer to as the "Common" were by far the most commonly used, accounting for over 90% of ammunition used, and loaded. Commons and AAC were the most common ammuniotn used in all of the 1942 battles.
I also suggest you study some of the battles a little more closely....the Japanese nearly always engaged Allied ships with their 5 in guns....if they were not effective, the US (and allies) could have closed the range with impunity....Japanese tactics were to close as quickly as possible, so as to be outside effective US torp range, but within Long Lance Range, launch their own torps (effective out to 16000 yards), retire, reload, and go back to do it all again.
In summary, you seem to be trying to make your case by slanting the definitions and requirements of a fleet destroyer to suit your arguments. The Fletcher/Gearing/Somers series of fleet destroyers were far and away the best developed during WW II. They had what was without dispute the best main armament, best FC system, adequate ASW capability, adequate (barely) range, best ASW and AAW sensors, excellent AAW armament, adequate habitability, and good sea-keeping. They were capable of being built in large numbers, rugged, with adequate speed. Other destroyers may have been superior in one area or another, but none were better balanced for the role of multi-purpose fleet destroyer in the threat environment in which they found themselves.
Your argument relies on the idea of "fleet destroyer" being the same as "multi-purpose" destroyer". You are saying that I am slanting the argument, yet, the definition is what it is, nothing you or I can do about that
Some would argue that the ASW capability of USN DDs was not "the best", their torpedo armament was terrible, and their surface gunnery somewhat hampered by the Dual Purpose nature of the 5/38. HOWEVER, the 5/38 conferred other advantages to the American DD forces that far outweighed the disbenfits of the weapon