50 cal (high rate of fire) vs 20mm cannon (hitting power)

[yulzari]

It is interesting to note that the Finns, who had a variety of guns on a variety of fighters, dealt with these issues by concentrating on the quality of the pilots and firing at close distances. Not that they would have been averse to bigger guns on better fighters, if they could get them.

On a different tack. I don't recall the source but I do recall a quote that Commonwealth pilot training concentrated so much on airmanship and so little on marksmanship that it resulted in pilots who could miss the target from closer to the enemy and at better approaches. It probably could be applied equally to most air forces.

 

[A.G. Williams]

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.

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.

 

[davparlr]

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

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.

 

[varsity07840]

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.

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.

 

[varsity07840]

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.

Was that the gun or the mounts? Considering the problems with the P-51B/C, I'm thinking it was the mounts.

 

[Greyman]

Both, at that stage. More so the mounts though, by the sounds of it.

Got into it near the start of this thread:
Wildcat during the Battle of Britain

 

[A.G. Williams]

I don't have any data on .50 reliability except for some late-war figures which show that it experienced one stoppage every 4,500 rounds (the Hispano managed 1,500 rounds).

Looking at the history of the .50 Browning; the original M1921 was completely reworked in the early 1930s and emerged as the M2. So the gun was around for the best part of a decade before the US got involved in the war - which should have been plenty of time to sort any problems. However, the aircraft gun was modified in around 1940 to improve its rate of fire (from c.600 rpm to 800 rpm IIRC) and possibly this increase affected reliability until this was sorted. It generally seems to have been regarded as reliable once combat got going.
.

 

[Shortround6]

I believe that the M2 aircraft gun had the belt pull (force exerted on the belt by the feed pawls) doubled at some point in 1940 or 41, this may have been in conjunction with the increase in rate of fire done at the same time. There was no change in nomenclature for either change. The increase in belt pull should have solved some (but not all) of the feed problems.
Guns in the field could be converted with suitable parts, how long that took to accomplish is not reported so far.

 

[Greyman]

I think it's extremely difficult to put exact numbers on overall reliability of weapon X or Y.

Stoppages rates could vary significantly:

1. over time -- eg: VIII Fighter Command Lightnings went from about 1750 rounds (.50-cal) per stoppage at the beginning of 1944 to about 5750 rounds by the middle of the year.
2. between installations -- eg: VIII FC Mustangs had a stoppage rate of about 750 rounds per -- while the Thunderbolts were about 1750 per (spring 1944)
3. between units -- I can't find the document anywhere at the moment, but I have an RAF report on fighter operations in Tunisia (I think) that noted the importance of weapon maintenance in this regard, giving figures for the average Hispano stoppage rate and the rate of the worst squadron in this respect. The difference was significant.

 

[davparlr]

- which should have been plenty of time to sort any problems.
.

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.

 

[Koopernic]

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 versus average recoil force. Presumably the heavy recoil of the Hispano due to the mechanism and heavy round required strengthening of the aircraft spars

It also occurs to me that the optimal configuration or armament may have been mixed 20mm ie Hispano V plus MG FFS or FFL in parts of the airframe less suited. Hispano V with MG151/20 might even work. Would a Typhoon have been able to carry 6 MG151/20 or Oerlikon in Lieu of 4 Hispano or a mixture?

Did the Germans ever become serious about the C30/C38 on aircraft? Closest they seem to have gotten was the MG213 which seems to have almost the same parameters.

 

[A.G. Williams]

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

The Hispano was a powerful gun which did not have an integral support (as a motor cannon, the support was supposed to be provided by bolting the gun to the engine). The peak recoil blow was considerable (the massive 57mm Molins aircraft gun carried by the DH Tsetse had a very long recoil movement, and a peak recoil blow similar to the Hispano's).

The Oerlikons enjoyed the benefit of their Advance Primer Ignition Blowback mechanism, which meant that the gun fired as the bolt was travelling forwards at maximum speed. The initial recoil blow therefore went into stopping the bolt's forward movement, and much of the remainder on pushing it back again, which produced a much smoother curve on the recoil graph with a low peak blow. These were ideal characteristics for aircraft cannon, making mounting the guns much easier.

Mixing different types of cannon in one installation was an unwelcome complication, not just in terms of logistics but also ballistics. The Luftwaffe did field some Fw 190 with MG 151/20 in the wing roots and MG-FF in the outer wings, but I gather that the MG-FFs were often removed.

 

[buffnut453]

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"?

 

[A.G. Williams]

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.

 

[buffnut453]

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.

 

[PAT303]

Quite 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.

 

[A.G. Williams]

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.

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.

 

[A.G. Williams]

Quite 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.

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

 

[tomo pauk]

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.

Do we have some quantification about the number of bullets fired before the guns were rendered useless? Eg. when Soviets tested the Beresins' HMG, the guns were good for 10000-12000 rounds fired.

 

[PAT303]

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

The Oz made 20mm ammo was terrible, a new factory was set up at St Mary's on the outskirts of Sydney and it was way out of spec.