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You have made some very good points, not picking the Oerlikon can go up there with not fitting aux fuel tanks to Spitfires as two ''shakes my head'' moments regarding the RAF.
Interestingly, both the F4F-3 and the P-51B both had four .50s and were replaced by six .50s configuration, F4F-4 and P-51D. One possible reason was reliability. There seemed to be a common complaint about jamming of the .50s. The loss of one or two guns on a four gun set certainly would be more problematic than the loss of one or two guns on a six gun set. The P-51B was certainly a successful aircraft and fought a good portion of the war, I suspect even to the end of the war. The F4F-4 had less ammo available thus shorter firing time.I think there is room here for a few more variables. First, if instead of 6 x .50s one went with 4 but increased the ammo it would give a commensurate increase in trigger time. Second, I think in combat there is a "it depends". Reliability would be number one, or in other words knowing that when I squeezed the trigger that the guns would work. I would take reliability as my number one choice, then increased rounds count (longer trigger time / Mk14 type gunsight?) until I became a confident shooter, then would switch to heavier caliber.
Food for thought.
Cheers,
Biff
The problem with the P-51B/C was that the guns were angled in the bays, causing feeding problems. The six gun F4F-4 was an off spring of a FAA requirement for folding wings and six guns. The six guns with less ammo was almost universally condemned by veteran pilots.Interestingly, both the F4F-3 and the P-51B both had four .50s and were replaced by six .50s configuration, F4F-4 and P-51D. One possible reason was reliability. There seemed to be a common complaint about jamming of the .50s. The loss of one or two guns on a four gun set certainly would be more problematic than the loss of one or two guns on a six gun set. The P-51B was certainly a successful aircraft and fought a good portion of the war, I suspect even to the end of the war. The F4F-4 had less ammo available thus shorter firing time.
Was that the gun or the mounts? Considering the problems with the P-51B/C, I'm thinking it was the mounts.When the British tested the .50BMG in the Martlet they jammed immediately, they later found out the Americans test fired them with the planes flying straight and level, in 1940 the .50BMG was worthless.
Yep, the squadron that had cannon armed Spitfires in the BoB, I think it was 606 Squadron demanded to be re-equipped with .303 armed fighters because the Hispano's didn't work.
Pre-war validation of U.S. combat equipment seemed to be lacking, probably due to lack of adequate funding. Most noted was the torpedo fiasco, but there was also issues with the radios, certainly with the Navy, where critical information did not reach the right place at the right time (probably poor procedures aided this problem), in addition to the .50 cals. Of course this happens with even more modern equipment, like the M-16, unfortunately people die when equipment does not work properly. And, it's difficult to test equipment to actual combat levels. Also, even today it seems that too many lessons are learned from dead bodies instead of equipment testing like in the 737Max. Airbus had similar failures only lucked out due to altitude when failures occurred which allowed crew to adapt. Ironically, had an Airbus software operating failure caused a loss of aircraft/people, the 737 fiasco may not occurred.- which should have been plenty of time to sort any problems.
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For clarification, there were three different sizes of 20mm Oerlikon aircraft guns for sale in the mid-1930s:
1. FF (20 x 72RB ammo, weight 24 kg; RoF 520 rpm, MV 600 m/s)
2. FFL (20 x 102RB ammo, weight 33 kg, RoF 500 rpm, MV 750 m/s)
3. FFS (20 x 110RB ammo, weight 39 kg, Rof 470 rpm, MV 820 m/s). This was a lightweight version of the famous Type SS AA gun extensively used by the RN and USN in WW2.
In my view, the MV of the FF was too low. It's a bit of a toss-up between the FFL and FFS; I prefer the FFL for the RAF because of the trajectory matching with .303 and to keep the size and weight down as much as possible. For the US (and especially the USN) the FFS would make more sense because it used the same ammo as the AA gun, and the velocity was a closer match with the .50 BMG.
In fairness to the RAF, when they made their choice of the Hispano in 1935, the contemporary Oerlikon it was compared with was relatively heavy and slow-firing, as it was the French HS 9 which weighed 48 kg and fired at 400 rpm; the prototype HS 404 weighed 50 kg and fired at 700 rpm (it also had a higher MV at c.880 m/s), so looked much better. Shortly afterwards, Oerlikon completely revamped their cannon range to reduce weight and increase the rate of fire as detailed above; while the HS 404 rate of fire was dropped to 600 rpm in the interest of reliability. So there was suddenly not such a big difference between the performance of the HS 404 and the significantly lighter FFS; and the API blowback mechanism of the Oerlikon had much smoother recoil and did not suffer from the installation problems of the Hispano.
So it could be argued that the RAF was rather unlucky with their timing; they urgently wanted a 20mm cannon and the HS 404 seemed to be the most promising one on the market at that time. A few months later and the Hisso's advantage had virtually disappeared.
Is there a figure of merit to measure peak recoil force? Presumably the heavy recoil of the Hispano due to the mechanism and heavy round required strengthening of the aircraft spars
It generally seems to have been regarded as reliable once combat got going..
Rather depends on your definition of that timeframe. I'm sure the P-40 pilots in the Philippines, Buffalo pilots in Singapore, F4F pilots thru Q3 1942, and P-51 pilots well into 1943 would have preferred more reliable weapons than they experienced. Pretty much every wing installation of 50cals seems to have been problemmatic for many months after the US entered the war, and in some cases well into 1943. Which timeframe do you consider as when "combat got going"?
Interesting. In the reading that I've done about aircraft guns, I have come across exhaustively detailed accounts of the problems of the Hispano and the continuing efforts to correct them (and also about the 37mm M4 ejection and how that was tackled), but virtually nothing about the Browning, apart from the problem with the canted wing installation which proved unreliable.
Lundstrom mentions persistent problems with wing 50 cals into Q3 of 1942 in "The First Team". Various books about the defence of the Philippines mention issues with wing 50cals in P-40s. The Buffalo's issues are well-known (but tend to be highlighted, whereas the issues with other aircraft tend to be ignored).
Jamming belt feeds and inadequate gun solenoids appear to be the most common issues, but it's surprising (to me, at least) that they persisted so long. I find it truly odd that the P-51, which was a relative late-comer to the party, still had problems with 50cal wing installations.
In what respect was the ammo out of spec? The RAF corrected the main problem of light firing pin strikes, by shortening the chamber. See: Modifications and Attempts at StandardizationQuite a few RAAF pilots complained about the .50's in the P40's we received because of jamming, and a lot more about the HIspano's because of out of spec ammo.
Thanks for that. I suppose one factor was that aircraft guns tended to be tuned to give the highest performance for the least weight, and were often operating on the ragged edge of reliability. The Soviets, for instance, worked out that the life of an aircraft gun in combat was very short, so they built them to last only just long enough.
In what respect was the ammo out of spec? The RAF corrected the main problem of light firing pin strikes, by shortening the chamber. See: Modifications and Attempts at Standardization