Rn vs IJN

Shortround6 said:

Well, there is leading it and there is leading it by 25 times it's own body length
(Swordfish at 2,000yds doing 130mph)

Picket ships were supposed to be early warning. They were not the outer ring of the AA defenses.
And a lot depends on the navy and the timing (which year).

RN destroyers could not use their own 4.7 in guns to shoot at aircraft flying over them. Max elevation was 40 degrees for all the pre-war stuff. They could shoot at planes that were around 10,000ft high over ship several miles away. At least that was the theory. Now try it when the planes are coming from the opposite side of the target ship. Yes they could shoot at lower flying planes (torpedo bombers, low level bombers). Against dive bombers or any plane doing an above 40 degree attack most of the old (pre Tribal class) British destroyers only had either two 2pdr guns (the old ones) or two quad .5in mounts until 1939/40. Ability to present massed high angle (over 40 degrees) from destroyers was pretty much non-existent.
No data sharing
Tribals got a quad 2pdr and were pretty hot stuff. And they got the two quad .5in MGs, which were replaced/supplemented by 2-4 20mm guns.
In spite of using a better AA director for the main guns (for helping out their buddies) all of the Tribals had one twin 4.7 mount taken off and replaced by by a twin 4in mount for high angle fire.

US didn't get into war, as we all know, until the end of 1941. US air defense practice took a while to evolve and the US was actually ahead of everybody else. What the US was doing in 1944/45 was way different that what everybody else was doing in 1939-42.

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The RN was rearming older cruisers and destroyers with the twin 4in HA gun and they had a large number of 4in armed escort destroyers and sloops under construction. The big fly in this ointment was that upon reading USN Action Reports from 1942 (and Lundstrom's volumes about the naval airwar in the Pacific), it becomes apparent that 5in, destroyer, cruiser, carrier and battleship AA was ineffective against aircraft with the vast majority of AA kills coming from the close range autocannon. USS Yorktown's final Action Report recommended replacing carrier 5in AA with 40mm:

• (e) Replacement of 5"38 caliber guns, 1.1" guns and 50 caliber machine guns, by a large number of 40mm automatic guns. While smaller caliber automatic guns have proven effective at short ranges, their range is too short to offer effective opposition to attacking planes prior to delivery of their attack. 5"38 caliber guns are very effective at long ranges and should be retained in ships which are used as anti-aircraft screening vessels.

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The recommendation to keep 5in guns for long range AA coincides with the RN's decision to keep 4.7in destroyer armament at 40 deg elevation and to fit destroyers with an AA FC system. However, even at longer ranges 5in AA kills were far and few between.

Does that include 5"38 guns with VT shells or was this before they were introduced?

There was a lot of back and forth going on and lot of it was either test or theory and not actual combat.

British were laying down the L class in 1938-39 so planning began ???


Guns elevated to 50 (?) degrees, They were actually pretty much a waste of time and effort. In part because they only armed 12 destroyers with them (and nothing else) so they are little more than a foot note. The whole thing sounds good on paper but didn't work in practice. The ammunition hoists in the center of the mount (not turrets) didn't turn so if the guns were firing at 90 degrees the ammunition hoists were still lined up with the center of the ship. The mounts used manual elevation, did not have power rammers and while traverse was powered it was slow. Basically they weren't any better and were probably worse than the older 40 degree elevating mounts. Four of the ships had four twin 4in mounts and were practically little AA cruisers They also worked rather well in close range night battles with their high rate of fire. Again, practical experience vs theory.

Swiped from a foot note on Wiki.
Hodges, Tribal Class Destroyers, p32: Diagram of High Level Bomber Attack: A 240mph target, at 12 thousand feet altitude could expect to be under for fire about 75 seconds, from the time it enters the effective range of the HACS until it flies to within the minimum range of a 5.25 gun elevated to 70 degrees. A Tribal class destroyer with 40-degree elevation guns would be able to engage the same target for about 37 seconds.

Now we are comparing two different guns here and the 5.25 gun does have a much longer range than the 4.7 gun but the 4.7 gun in question only has a 10-12 rpm firing rate so basically you get about 8 shots per barrel at best as the plane goes from max range to the inner range where the gun can't elevate high enough anymore until the plane is departing.
Also bombing from 12,000ft actually didn't work so that bombing profile went away.
I am wondering about the steep dive approach used by the Swordfish and Albacore???
Go in over the "top" of the low angle enemy guns, dive down and level out close to the ship where the enemy has to reacquire the attacking aircraft and only has enough time for 2-3 shots?

This seems to reinforce a couple of points I was making:

(e) Replacement of 5"38 caliber guns, 1.1" guns and 50 caliber machine guns, by a large number of 40mm automatic guns. While smaller caliber automatic guns have proven effective at short ranges, their range is too short to offer effective opposition to attacking planes prior to delivery of their attack. 5"38 caliber guns are very effective at long ranges and should be retained in ships which are used as anti-aircraft screening vessels.

But that is just one report from one particular moment. The notion that the American AAA wasn't shooting down any IJN planes is absurd, the Japanese were taking terrible losses to AAA, and they don't always know when a plane is hit by the large caliber, medium or small caliber weapons. That is just one report among many across a long war.

Wild_Bill_Kelso said:

I seriously doubt it. Armor doesn't make an aircraft immune to bullets, especially heavy machine gun bullets.

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The IL2 was very well protected and very resistant to even 20mm cannon, so a 12.7 bullet would struggle. I don't see why the Hs129 would do any worse

This recommendation that was in the USS Yorktown Action report

e) Replacement of 5"38 caliber guns, 1.1" guns and 50 caliber machine guns, by a large number of 40mm automatic guns. While smaller caliber automatic guns have proven effective at short ranges, their range is too short to offer effective opposition to attacking planes prior to delivery of their attack. 5"38 caliber guns are very effective at long ranges and should be retained in ships which are used as anti-aircraft screening vessels.

Reminded me that the late war/early post war Colossus Class carrier had an all LAA defence, maybe someone learnt the same lesson.

SaparotRob said:

Does that include 5"38 guns with VT shells or was this before they were introduced?

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5in VT didn't become available to the USN until early 1943, so beyond the scope of Lundstrom's books. The RN began to receive VT in late 43/early 44.

Glider said:

The IL2 was very well protected and very resistant to even 20mm cannon, so a 12.7 bullet would struggle.

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"Very" resistant doesn't mean, for either aircraft, that if wings, tail, even control surfaces are blown off, it will almost always crash. Surviving a few shell strikes from a pursing fighter is not the same thing as surviving a fusilade from a multi-barrel antiaircraft weapon.

Just as a little reality check - the Soviets lost over eleven thousand Il-2 Sturmovik, the armor is nice but it only goes so far. I don't think a whole lot of those were shot down by 4.7 or 5" naval guns either.

We are going to have to agree to disagree on this. A 12.7 bullet doesn't have an explosive warhead and is highly unlikely to blow anything off. It will of course damage anything that it hits but it is more likely to cause a hole as it goes in and another as it goes out, unless it hits something. A 20mm shell will do far more damage and it will penetrate more armour in the first place.

Its also worth remembering that depending on the 20mm, the rate of fire from a 0.5 M2 wasn't that much more than some 20mm so the concept of a stream of 12.7mm vs the odd 20mm isn't valid.
Its approx. a 3 to 2 ratio in rate of fire and I would bet on 2 x 20mm vs 3 x 12.7 any day

Pre-war the idea with heavy AA was to predict where the target aircraft would be and fire the rounds in such a manner as to have them arrive where the target was at the predicted time/position. In actual use it was found that the predictive systems were relatively ineffective as they could not produce an accurate firing solution fast enough. While much hoorah has been made about the US Mk 37 FC directors and computers, they really did not do much (if any) better than the older systems - until the proximity fuze and integrated radar came along. After that they were probably the best of the war-time systems, and remained effective through at least the mid-1950s.

Before the proximity fuze the most effective method was the barrage system. The RN worked the predominant methodology out in the early-war and the USN adopted aspects for the most part after early-1943. Apparently HMS Victorious (USS Robin) demonstrated the method for the USN at some point and the higher-ups were somewhat impressed. The idea was to fire a few rounds from the heavy AA guns at a given range, timed for the fuzes to set off the first rounds in front of the incoming aircraft, forcing the aircraft to either fly through the succeeding rounds or break off the attack. The guns would fire at the farthest preset range, either a set number of rounds or until it was observed that the incoming aircraft had passed through the barrage, whereupon the range was reduced to the next shorter range, etc. A sufficient number of rounds had their timed fuzes preset and kept at/near the mount to go through the process at least once - sometimes more than once. This reduced the problem of the systems in use early-war not really being able to predict the position of the incoming aircraft fast enough.

If dive bombers were in play the escort ships would also fire an umbrella barrage over the protectee (if they could do so safely), forcing the incoming aircraft to fly through the barrage - hopefully disrupting the attack and screwing up the aim of the dive bombers. This was the method used during Operation Pedestal for one example.

When proximity fuzes came along it made direct fire by the heavy AA guns - out to similar shorter ranges used for the barrage method - far more effective than before, with few corrections needed. The previous barrage system - with modifications - was still used at medium ranges.

It was not until integrated radar FC systems matured that the proximity fuze became really effective at longer ranges. The post-war FC systems on RN Leander class frigates, for example, were considered to only need 2 salvos from their twin 4.5" turret to achieve a 80% chance of a kill vs an aircraft attacking the firing ship. Crossing targets still presented a more difficult firing solution but much less so than in WWII when it was almost a matter of luck or individual crew talent rather than method.

Glider said:

We are going to have to agree to disagree on this. A 12.7 bullet doesn't have an explosive warhead and is highly unlikely to blow anything off. It will of course damage anything that it hits but it is more likely to cause a hole as it goes in and another as it goes out, unless it hits something. A 20mm shell will do far more damage and it will penetrate more armour in the first place.

Its also worth remembering that depending on the 20mm, the rate of fire from a 0.5 M2 wasn't that much more than some 20mm so the concept of a stream of 12.7mm vs the odd 20mm isn't valid.
Its approx. a 3 to 2 ratio in rate of fire and I would bet on 2 x 20mm vs 3 x 12.7 any day

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I'm a bit baffled by this - did you think I was arguing, at any point, anywhere, that a 12.7mm was superior to a 20mm ? Because that seems to be the point you are trying to refute here.

I never made such a claim. I noted that the quad 12.7mm was quite effective, historically.

However when you claim that a 12.7mm machine gun (or a few of them) can't blow off the wing of an aircraft, no we can't just "agree to disagree". I'm sure you have seen gun camera footage from WW2 in which US fighters are shooting Axis aircraft and their wings break off? Did you forget? Do I really need to go grab some and post it here?

No WW2 aircraft was immune to heavy machine gun, let alone quad heavy machine gun fire. They mitigated hits with armor somewhat, but that only slightly reduced the loss rate. Pilots did their best to avoid being hit - that was the only way to stay safe, if you could manage it. Part of the purpose of any kind of AAA fire is to drive enemy aircraft away, to rattle them, to force them to make mistakes and to break off engagements. All of this and more happened as the direct result of all kinds of AAA gunfire, from "little" .30 caliber machine guns all the way up to 128mm heavy AA guns. But the 12.7mm definitely did tear up, blow up, rip apart, blow holes through, set on fire, cause to explode, and generally wreck all kinds of Axis aircraft, as in tens of thousands, whether from an MGMC-16 on the ground, or in the defensive guns of a B-17 or B-24, or in the wing mounted guns on a P-51, Hellcat, Corsair, P-47, P-40, or Wildcat (and later in the war, for some reason British fighters too). And I'm sure the heavy machine guns on many US Navy ships did as well.

Wild_Bill_Kelso said:

I think AA defense was typically layered. Heavy machine guns (and 20mm cannon) have the role of hitting lower and closer targets. 3", 5", 88mm, 90mm etc. are for much more distant (and usually high flying) targets. 57mm, 40mm and 37mm (etc.) are in between.

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I think the big distinction is between heavy AA and the others. Heavy AA was dependent on fragmentation with time fuses (yes, later on proximity fuses) and had a relatively slow rate of fire. So they were not really usable at short range. In contrast, light and medium AA were in many ways similar in that they relied on hitting the target, and were effective down to point blank range.

Obviously the mutli-barrel weapons greatly increased the viability of these lighter weapons. Both the quad 50 and the quad 20mm were considered quite dangerous by pilots, I can promise you that.

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Once again, I'm not disputing that. But that doesn't prove that 40mm wasn't even more dangerous.

I suspect things like production limitations etc kept the light AA in use instead of being replaced by the more effective medium AA. In other words, the choice wasn't between replacing light AA with medium AA, but between keeping light AA and nothing.

ThomasP said:

40mm single Mk IX 'Boffin' mount
View attachment 742341

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The "Boffin" was a Mk.V or Vc twin 20mm Oerlikon mount with the guns replaced by a single 40mm gun and converted as a rush job in 1944/45. Elevation was limited to -10 to +70 degrees. That limitation was one reason why Commonwealth ships in 1944/45 shipped a mix of these and the hand worked land service Mk.III which had a -5 to +90 degree elevation.

In March 1945 the Mk.VII mount was ordered into development as a properly sorted powered single 40mm mount similar to the "Boffin" but with +90 degree elevation.

The Mk.IX was a postwar development with electric instead of hydraulic drive. It served in the RN at least into the 1980s.

The latter pair were never referred to as the "Boffin" AFAIK.

z42 said:

I think the big distinction is between heavy AA and the others. Heavy AA was dependent on fragmentation with time fuses (yes, later on proximity fuses) and had a relatively slow rate of fire. So they were not really usable at short range. In contrast, light and medium AA were in many ways similar in that they relied on hitting the target, and were effective down to point blank range.

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Well, I think there is a distinction between all three, or four (128mm is kind of in a 'super-heavy' category all it's own [EDIT: I should say "land based" 128mm here]. Today there are still three categories: rapid firing 3" or 5" guns, medium caliber (like 30mm or 25mm chain guns) and light caliber (20mm phalanx / CIWS) type guns on major warships... plus typically long, medium and sometimes also very short range missile systems. And now some are using energy weapons as well I think.

z42 said:

Once again, I'm not disputing that. But that doesn't prove that 40mm wasn't even more dangerous.

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If I was trying to shoot something or someone, I'd of course rather have a 20mm than a 12.7mm, and I'd rather have a 40mm than either of the smaller options. It's certainly going to have better range and hit harder (as will 37mm or 25mm). But I don't think that means the lighter gun mounts are categorically inferior across the board. For land army / vehicle based, I can tell you that the Bofors 40mm mount was not light or easy to deploy. The US tried to mount these on vehicles, during WW2 they finally built the M19 MGMC which was basically a twin Bofors mounted onto a Stuart tank chassis, too late for the war though they used them a bit in the Korean War. Then they had something called the M42 "Duster" which was only marginally successful, and then the infamously unsuccessful M247 "Sergeant York" also of the Vietnam era.

The British mounted a Bofors into a Crusader tank, which became the Crusader III, AA MK 1, but they seemed to prefer the III, AA MK II and Mk III, which had twin Oerlikon 20mm.

The US also had the MGMC M 15 which had two 12.7mm and a 37mm, and it worked, but they ended up phasing it out in favor of the M 16 which had the four 12.7mm. Later they developed the Vulcan 20mm which the US mounted on M113 APCs and LAV type wheeled vehicles I think. The Vulcan 20mm is a very impressive weapon, the problem I saw with it is that it uses up an entire APC full of ammunition in about 1 minute.

Later more modern AA vehicles had 23mm (like the very successful Russian ZU-23 etc. types), 25mm (like the Chinese Type 95), or 30mm (South Korean K30, Soviet 2K22), or 35mm (German Gepard and several others). Of course the missions have changed - the targets are largely helicopters, sometimes missiles, and now increasingly drones, though they will also still be used against low flying CAS aircraft like A-10s and SU-24s etc. There are also a few with bigger guns than a 40mm, like the Soviet ZSU -57 (with twin 57mm AA guns). We also have MANPAD missiles and quite effective medium and long range SAMs, but there is still a niche for gun defence, albeit a smaller one. Now days defense against drones, rockets, and guided missiles, is particularly vital and the Israeli's started down that (I think going to be prove very long) road with their TROPHY point defense system for tanks.


At very close range, a 40mm gun is not quite as quick to track and 'acquire' targets as a 20mm or some kind of machine gun. Maybe that is why it does not seem to have ultimately been the ultimate AAA gun over all other options. Or maybe it has to do with ammunition capacity or storage, or the extra weight. I don't know from the engineering side, but looking at the history it seems that as good as it was, the 40mm AA gun was phased out, often in favor of lighter guns such as 23, 25 and 30mm (35mm is fairly close though, still popular and probably comparable). I think the Drone era is going to see a lot of new changes being made.

z42 said:

I suspect things like production limitations etc kept the light AA in use instead of being replaced by the more effective medium AA. In other words, the choice wasn't between replacing light AA with medium AA, but between keeping light AA and nothing.

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I don't think that was necessarily the case. I agree with the premise that 40mm Bofors guns were probably ideal air defense for larger ships in WW2 (though they may have also wanted a layer of at least some lighter guns). Smaller ships (destroyers down to PT boats) are definitely going to want some lighter AA guns as well.

It should be noted that 5" = 127mm. Why should they be "heavy" but German 128mm be "super-heavy"?

Hey EwanS,

Your are correct. Thank you for pointing out my error with the designation re the 'Boffin' - I have corrected my post.

Thumpalumpacus said:

It should be noted that 5" = 127mm. Why should they be "heavy" but German 128mm be "super-heavy"?

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Ah, that's a good point, my mistake.

I was thinking in terms of ground based guns. 3", 88mm (3.4") and 90mm (3.5") guns were kind of the heavy guns on land, big but still small enough to be moved around on trucks and wrestled into position by crews, whereas the 128mm / 5" guns (like the Flak 40, US M1 / 120mm) were basically for fixed installations (and very high altitude targets, up to 50 or 60,000 ft).

I think there is a difference between a motorized 4.5" or 5" gun on a ship, which can be quickly trained to a target, vs one which is manually aimed like the 12.8 cm Flak 40

Some ships also had bigger manually aimed guns early in the war as well, though I'm not sure if these were AA or just anti-ship guns.

Wild_Bill_Kelso said:

Ah, that's a good point, my mistake.

I was thinking in terms of ground based guns. 3", 88mm (3.4") and 90mm (3.5") guns were kind of the heavy guns on land, big but still small enough to be moved around on trucks and wrestled into position by crews, whereas the 128mm / 5" guns (like the Flak 40, US M1 / 120mm) were basically for fixed installations (and very high altitude targets, up to 50 or 60,000 ft).

I think there is a difference between a motorized 4.5" or 5" gun on a ship, which can be quickly trained to a target, vs one which is manually aimed like the 12.8 cm Flak 40

Some ships also had bigger manually aimed guns early in the war as well, though I'm not sure if these were AA or just anti-ship guns.

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Got it now, we just had different regimes of ops in mind.

Thumpalumpacus said:

Got it now, we just had different regimes of ops in mind.

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yeah my whole categorization system for AA guns is pretty much rubbished by your very relevant point though lol