USAAF 0.60-caliber Machine Gun????

[Colonial-Marine]

I don't know what US experiments occurred with .50 caliber HEI ammunition but late in the war a .50 caliber "super incendiary" cartridge was introduced that carried a lot more compound than the common API cartridge. It didn't get to see too much use during the war, but I think it left a positive impression. The 13.2mm FN Brownings could fire what is supposed to have been a rather effective HEI cartridge too.

There are two things I think would have really benefitted USAAF (and USN aviation) armament during the war.

#1 would be if FN had gotten plans and guns for the 13.2mm Browning over to the UK before the Germans took Belgium. I believe the UK and France had already agreed to adopt the weapon, but events had made that irrelevant. If the UK did get examples however it's possible the USA could have put into production an improved .50 caliber machine gun with a higher rate of fire, closer to that of the AN-M3 which historically was only introduced in the final days of the war. The US certainly wouldn't switch from .50 caliber (12.7mm) BMG to the 13.2mm cartridge, but that difference is pretty minimal. Perhaps examples of that 13.2mm HEI would aid in the US developing similar projectiles in .50 BMG.

#2 would be fixing the production issues which cause such difficulties with US-built 20mm Hispanos. That would probably also greatly aid in making the .60 caliber T18 variant of the Hispano function somewhat reliably. Whether or not that .60 caliber option would have any worthwhile applications is its own question, however. In most cases 20mm is the better option considering the weight of the guns and ammo would be about the same.

 

[HoHun]

Hi Scott,

Yeah, they didn't face enough heavy aircraft that they had to try to stop with .30cal or .50cal.

Which is why the Russians went to 23mm or 37mm, the Germans to 20mm and then 30mm, and the UK to 20mm.

My impression is that historically, it was a bit more complicated, largely due to institutional inertia. If I understand it correctly, the USAAF did actually expect to use fighters in a defensive role, specifying interceptors in the shape of the P-38 and the P-39 (and a bomber destroyer in the shape of the Airacuda), which were to be armed with cannon. That the P-38 got a 20 mm cannon was sort of a downgrade with regard to the original design intentions, which seem to have envisioned the use of cannon between 23 mm T1 up to the 37 mm M4 also used by the P-39.

That the USAAF ended up with largely 0.50" heavy machine gun-equipped fighters was at least partially by accident, probably because the P-40 as a development of the P-36 sort of outshone the P-39, which generated an order for North American to produce an improved P-40, which (I'd speculate) received with the same armament as the P-40 due to the USAAF being a burnt child with regard to the Hispano 20 mm cannon.

I wouldn't consider 12.7 mm HMGs a weapon optimized for anti-fighter combat either. The German assessment of combat against modern aircraft with a load-bearing metal skin was that destroying said skin was the quickest way to cause an immediate failure, while purely kinetic projectiles of any calibre could only be effective against the relatively small critically vulnerable parts of the aircraft, leading to much lower chances for an immediate kill.

Thus, their use of 20 mm cannon was not in any way motivated by a desire to shoot down bombers specifically. The Bf 110, which was envisioned as a long-range fighter that cleared the sky around the Luftwaffe bombers from enemy fighter, was armed with cannon, because these were seen as the best air-to-air weapons.

(The 30 mm MK108 on the other hand really seems to have been developed as an anti-bomber weapon, though it really looks like the design brief was, "make it just like the MG FF/M, only bigger!" ;-)

Regards,

Henning (HoHun)

 

[drgondog]

Hi Scott,



My impression is that historically, it was a bit more complicated, largely due to institutional inertia. If I understand it correctly, the USAAF did actually expect to use fighters in a defensive role, specifying interceptors in the shape of the P-38 and the P-39 (and a bomber destroyer in the shape of the Airacuda), which were to be armed with cannon. That the P-38 got a 20 mm cannon was sort of a downgrade with regard to the original design intentions, which seem to have envisioned the use of cannon between 23 mm T1 up to the 37 mm M4 also used by the P-39.

That the USAAF ended up with largely 0.50" heavy machine gun-equipped fighters was at least partially by accident, probably because the P-40 as a development of the P-36 sort of outshone the P-39, which generated an order for North American to produce an improved P-40, which (I'd speculate) received with the same armament as the P-40 due to the USAAF being a burnt child with regard to the Hispano 20 mm cannon.

I wouldn't consider 12.7 mm HMGs a weapon optimized for anti-fighter combat either. The German assessment of combat against modern aircraft with a load-bearing metal skin was that destroying said skin was the quickest way to cause an immediate failure, while purely kinetic projectiles of any calibre could only be effective against the relatively small critically vulnerable parts of the aircraft, leading to much lower chances for an immediate kill.

Thus, their use of 20 mm cannon was not in any way motivated by a desire to shoot down bombers specifically. The Bf 110, which was envisioned as a long-range fighter that cleared the sky around the Luftwaffe bombers from enemy fighter, was armed with cannon, because these were seen as the best air-to-air weapons.

(The 30 mm MK108 on the other hand really seems to have been developed as an anti-bomber weapon, though it really looks like the design brief was, "make it just like the MG FF/M, only bigger!" ;-)

Regards,

Henning (HoHun)

With one exception I feel you are dead on.

NAA proposed installation of 4x20mm in March 1940 during the P-509 Specification refinement - at BPC request, but NAA was halted by Materiel Division because the Hispano license had not produced enough to 'divert to a British project'. When the P-509 Specs morphed into NA-73X, then NA-73 the 20mm was dropped altogether. At that time the Tomahawk II and IIB were pre- six gun wing. I suspect BPC was a factor in up-gunning the P-40, along with SS tanks.

The original armament proposed by NAA for the A-36 in November 1941 was also 4x20mm, along with various combinations of 20mm and 37mm cannon. The Army forced the 6x50 final decision. What was curious to me is that Brewster was able to recommend and then build and test fly the 20mm battery XA-32. There clearly were two factions within Materiel Division and then Materiel Command.

 

[Colonial-Marine]

They could have been better, but I think the common armament of 6x0.50" MGs was good against most fighters of WWII. Many Japanese fighters were lacking armor or even self-sealing fuel tanks, so they had little trouble dispatching those. Even their two engine bombers often lacked such survivability measures because of the extreme emphasis placed on range and performance. On many fighters even 4x0.50" caliber MGs was often adequate for the job.

Large Axis aircraft with four or more engines were pretty rare. The toughest targets that might be somewhat common would probably be bombers like the He-111 or Ju-88. There are more rare types like the He-177 too, but I think harsh language was enough to convince the engines on those to burst into flames. It would probably be frustrating trying to shoot down an Hs-129, but I don't know how much use those saw outside of the Eastern Front.

Generally speaking, American F4U Corsair pilots during WWII preferred the 6x0.50" MG armament because of the larger ammo load and the ability to send more bullets downrange in a few seconds of firing. But some appreciated the greater destructive power of the 4x20mm cannon option. By the time of the Korean War, I believe the Corsair pilots then openly preferred 4x20mm cannons. That probably reflects improvements to reliability that had been made and the difference in targets being engaged.

 

[HoHun]

Hi,

They could have been better, but I think the common armament of 6x0.50" MGs was good against most fighters of WWII. Many Japanese fighters were lacking armor or even self-sealing fuel tanks, so they had little trouble dispatching those.

Good point - P-38 ace MF Kirby told me once, if he his burst of fire hit an A6M squarely, the 20 mm cannon shells were "just the icing on the cake". But of course, the P-38 had the additional advantage of the 0.50" MGs were that they were concentrated on the centreline, which in my opinion makes the P-38's guns a lot more effective under most circumstances than the same number of guns in the wings. Roger Freeman's "Combat Record: Mustang" has some pilot quotes from the China/Burma/India theatre that show that the USAAF pilots there found the Japanese Army fighter tough at times, even pointing out that the armour they found on a downed Ki-43 was thicker than that on their own fighters, much to their surprise.

However, to get back on topic, it might well be that the USAAF considered the 0.60" HMG the "optimal fighter weapon". I haven't really read much about the motivation for going to an only slightly larger cartridge with a much more powerful propellant charge. Of course, if you have a weapon that has a very flat trajectory, you can in theory engage at very long ranges, so one could argue it would have been a good anti-bomber weapon too. However, the USAAF gunnery training material emphasized that the main range-limiting factor was dispersion, and as far as I can tell, dispersion in light-weight (aircraft use) tends to increase with muzzle velocity as well as with barrel length (though the two factors are obviously connected). So realistically, the high muzzle velocity would serve mainly to minimize the amount of lead required, which would be more important in combat against fighters, thus my initial suggestion that's what the 0.60" HMG was meant for.

Generally speaking, American F4U Corsair pilots during WWII preferred the 6x0.50" MG armament because of the larger ammo load and the ability to send more bullets downrange in a few seconds of firing.

Hm, if you have any material on that, I'd be interested. During the Joint Fighter Conference (covered by a book of the same name), the Navy representative pointed out that the advantage of the Hispano 20 mm cannon was that it was capable of longer sustained fire and thus was better for strafing. The 0.50" M2 could only fire short bursts before the barrel overheated, dispersion increased wildly, and finally the barrel "burnt out". Here's a link to a video that shows gun camera footage of such an instance and provides a brief explanation:

View: https://youtu.be/sdDShOjpWm4?t=784


[Edit: Added video time stamp, 784 s resp. 13:04 min]

As this effect probably scales with something like the ratio of propellant energy to barrel inner surface area, I wonder if this issue might have been even more pronounced for the very powerful, but not much larger 0.60" machine gun.

Regards,

Henning (HoHun)

 

[drgondog]

Hi,



Good point - P-38 ace MF Kirby told me once, if he his burst of fire hit an A6M squarely, the 20 mm cannon shells were "just the icing on the cake". But of course, the P-38 had the additional advantage of the 0.50" MGs were that they were concentrated on the centreline, which in my opinion makes the P-38's guns a lot more effective under most circumstances than the same number of guns in the wings. Roger Freeman's "Combat Record: Mustang" has some pilot quotes from the China/Burma/India theatre that show that the USAAF pilots there found the Japanese Army fighter tough at times, even pointing out that the armour they found on a downed Ki-43 was thicker than that on their own fighters, much to their surprise.

However, to get back on topic, it might well be that the USAAF considered the 0.60" HMG the "optimal fighter weapon". I haven't really read much about the motivation for going to an only slightly larger cartridge with a much more powerful propellant charge. Of course, if you have a weapon that has a very flat trajectory, you can in theory engage at very long ranges, so one could argue it would have been a good anti-bomber weapon too. However, the USAAF gunnery training material emphasized that the main range-limiting factor was dispersion, and as far as I can tell, dispersion in light-weight (aircraft use) tends to increase with muzzle velocity as well as with barrel length (though the two factors are obviously connected). So realistically, the high muzzle velocity would serve mainly to minimize the amount of lead required, which would be more important in combat against fighters, thus my initial suggestion that's what the 0.60" HMG was meant for.



Hm, if you have any material on that, I'd be interested. During the Joint Fighter Conference (covered by a book of the same name), the Navy representative pointed out that the advantage of the Hispano 20 mm cannon was that it was capable of longer sustained fire and thus was better for strafing. The 0.50" M2 could only fire short bursts before the barrel overheated, dispersion increased wildly, and finally the barrel "burnt out". Here's a link to a video that shows gun camera footage of such an instance and provides a brief explanation:

View: https://youtu.be/sdDShOjpWm4?t=784


[Edit: Added video time stamp, 784 s resp. 13:04 min]

As this effect probably scales with something like the ratio of propellant energy to barrel inner surface area, I wonder if this issue might have been even more pronounced for the very powerful, but not much larger 0.60" machine gun.

Regards,

Henning (HoHun)

My father was not particularly fond of flak trains - from your YouTube reminder. The first image is a belly landing at Steeple Morden, cockpit retracted, with all hydraulics shot out, two 20mm hits in the aft fuse tank, several others knocking out O2 and intermittently flaming up on the return, tail wheel destroyed.

The second was a 37mm hit causing a snap roll near the deck.

Nov 29 excitement was his last mission, first tour, before returning to US for 30 day leave. He levelled 65th Wing planning for the assigned strafing attack when the weather forecast accurately predicted 2-3000 overcast (accurate) which enabled the German gunners to easily set fuses.

 

[HoHun]

Hi Bill,

The first image is a belly landing at Steeple Morden, cockpit retracted, with all hydraulics shot out, two 20mm hits in the aft fuse tank, several others knocking out O2 and intermittently flaming up on the return, tail wheel destroyed.

Thanks a lot for sharing the pictures! Looks like your father has the approach perfectly lined up despite all the damage done to his aircraft. If Clostermann's description of what it feels like to come in for a belly landing is any indication, that shot has captured a very intense moment!

The second was a 37mm hit causing a snap roll near the deck.

Wow, I wouldn't have thought a Mustang could fly on with that kind of damage, much less land in one piece to be be photographed!

Regards,

Henning (HoHun)

 

[MrTopaz]

There's another quote Tony provided which I am unfortunately unable to find, that stated that an X% increase in muzzle velocity will result in a Y% increase in probability of hit. With Y% being unreasonably high, in my opinion.

Bit of necromancy, but I might have stumbled across the values for X and Y that you recalled. In post 67 in this thread over at WW2aircraft.net a user known as Jabberwocky says (albeit without sources), “Someone had worked out that for deflection shooting that if you cut the flight time by 1/3rd then the chance of a hit went up by 400% for the average pilot,” which I would say conforms with your characterization of an unreasonably high increase in hit probability.

 

[HoHun]

Hi MrTopaz,

Bit of necromancy, but I might have stumbled across the values for X and Y that you recalled. In post 67 in this thread over at WW2aircraft.net a user known as Jabberwocky says (albeit without sources), “Someone had worked out that for deflection shooting that if you cut the flight time by 1/3rd then the chance of a hit went up by 400% for the average pilot,” which I would say conforms with your characterization of an unreasonably high increase in hit probability.

Thanks a lot!

That led me to this post, which looks like verbatim quote, albeit the source is only given as "One of Anthony Williams' books":

USAAF 0.60" Cannon

In one of Anthony Williams books he states "US research indicated that reducing the time of flight by one third would quadruple the hit probability, because most pilots seriously underestimated the amount of lead required in deflection shooting." Designing a cartridge and bullet to achieve...

ww2aircraft.net

"US research indicated that reducing the time of flight by one third would quadruple the hit probability, because most pilots seriously underestimated the amount of lead required in deflection shooting."

I checked "Rapid Fire" and "Flying Guns - World War II", which are the two books I have, and it doesn't seem to be in there (or mentioned in an unexpected place, though the indexes of both books appear to be quite thorough).

Tony technically seems to be a member of this forum, but he hasn't logged in for a year now. I don't seem to be able to @-add him here, so I guess that might be a planned absence ... I think he retired from his own forum as well a while back.

Regards,

Henning (HoHun)

 

[TomS]

I checked "Rapid Fire" and "Flying Guns - World War II", which are the two books I have, and it doesn't seem to be in there (or mentioned in an unexpected place, though the indexes of both books appear to be quite thorough).

Google Books says it's on page 51 on Flying Guns - Worlds War II

 

[HoHun]

Hi TomS,

Google Books says it's on page 51 on Flying Guns - Worlds War II

Thanks a lot - now I feel like a Neanderthal for having wasted my time fruitlessly browsing through an antediluvian paper book! ;-)

Too bad that Tony's book, while having a very extensive bibliography, doesn't use footnotes ... I'd really have loved to learn more about the background of this statement.

Regards,

Henning (HoHun)

 

[TomS]

Hi TomS,



Thanks a lot - now I feel like a Neanderthal for having wasted my time fruitlessly browsing through an antediluvian paper book! ;-)

Too bad that Tony's book, while having a very extensive bibliography, doesn't use footnotes ... I'd really have loved to learn more about the background of this statement.

Regards,

Henning (HoHun)

I only checked Google Books after having looked fruitlessly thought my copy of Flying Guns -- The Modern Era on the off chance it might have shown up there.

 

[HoHun]

Hi again,

I'd really have loved to learn more about the background of this statement.

["US research indicated that reducing the time of flight by one third would quadruple the hit probability, because most pilots seriously underestimated the amount of lead required in deflection shooting." ]

Thinking about this for a bit, I'd say it obviously can't be universally applicable, since in a no-deflection attack, neither the reasoning nor the numerical factor would make any sense.

As the numerical factor is quite substantial, I'd say the statement probably applies to a case in which deflection shooting is required, and at a range where this results in very low hit probabilities with the original low muzzle velocities.

Why not just fly closer to the target to increase hit probabilities? That can be explained for example by a reason to stay at the original long range, for example because the target is actually shooting back, like a bomber with flexible defensive armament would.

And here it might be useful to look at a US Army concept from the infantry context (based on my probably imperfect understanding - infantry is not my specialty): "Overmatch" describes the situation where the US force can use its firearms effectively to fight the enemy, while the shorter ranged firearms of the enemy can't yet be used to engage the US forces. Of course, that's the Army's preferred combat range.

So my guess for what the USAAF (still a branch of the Army back then) was trying to achieve with the 0.60" heavy machine gun, based on Tony's brief mention of the research they did, was to provide fighters with a firearm that allowed them to engage enemy bombers from an "overmatch" situation, in which the fighters would be able to destroy the enemy bombers without being exposed to effective return fire.

From USAAF gunnery training material, it's clear that the Army Air Forces thought any effective attack on a bomber would involve a pursuit curve (as the fighter would avoid approaching straight from the six o'clock, as that would give the bomber gunners a really easy shot), so the attacking fighter would inevitably have to engage in deflection shooting - which at long range was quite difficult indeed, and probably could in fact be materially improved by an increase in projectile velocity.

Interestingly, there was also some German research on the same topic, when the Luftwaffe considered the possbility that they would have to intercept fast jet bombers in the future, and their conclusion was that while increased projectile velocity was a clear advantage at longer ranges and larger deflection angles, this disadvantage of low-velocity weapons could be compensated for by the use of lead-computing optical gunsights. Ironically, sights of this type were first fielded by the RAF and USAAF, so if the German conclusions were in fact correct, the USAAF by the introduction of the K-14 computing sight had bascially made their motivation for requiring the 0.60" HMG obsolete.

Does that sound reasonable to you? It's a bit of a leap, supported only by very indirect and thin evidence, but after using the term "unreasonable" earlier, I felt sort of obliged to come up with an explanation that actually credits the USAAF with a rational approach, for a change ;-)

Regards,

Henning (HoHun)