Effect of the US produced HS.404R4M 20mm cannon being perfected pre-war?
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If they could have got the cannon to work reliably then I think we would have seen both Army and Navy fighters armed with it during the war.
As the .50 calibre M2 Browning proved adequate for the task that US aircraft were expected to perform its limitations, in terms of hitting power, were never really revealed. US aircraft never routinely had to engage formations of heavily armoured bombers like the Germans and Japanese, both of whom adopted ever larger calibre cannon for the task. The British had plumped for the 20mm cannon for much the same reason early in the war. For the most part the US fighters were engaging the much more fragile enemy fighters. This in turn meant that there was less urgency to develop reliable cannon armament, so we never saw it.
Cheers
Steve
As the .50 calibre M2 Browning proved adequate for the task that US aircraft were expected to perform its limitations, in terms of hitting power, were never really revealed. US aircraft never routinely had to engage formations of heavily armoured bombers like the Germans and Japanese, both of whom adopted ever larger calibre cannon for the task. The British had plumped for the 20mm cannon for much the same reason early in the war. For the most part the US fighters were engaging the much more fragile enemy fighters. This in turn meant that there was less urgency to develop reliable cannon armament, so we never saw it.
Cheers
Steve
I'm afraid that it would not bring too much. Maybe outfitting the P-39 with it and that's it; early US fighters were already over-burdened by hevy batteries (bar P-38 ).
Much more benefit would've been achieved with the V-1710 having a bigger supercharger
Much more benefit would've been achieved with the V-1710 having a bigger supercharger
That would have been a sensible use. The 20mm cannon of the 2nd TAF's Typhoons were by far their most effective weapon.
Cheers
Steve
- Thread starter
- #6
Shortround6
Major General
Just for the record the US spent a tremendous amount of money on heavy machine guns that went nowhere, ie they couldn't get them to work to acceptable level of reliability or durability.
from Tony Williams website:
Cartridges 7,8, 9 and 10 from the left were ALL experimental cartridges worked on in WW II. "15.2x114 (US .60" T17), 12.7x114 (US .50/60"), 12.7x120 (US .50" HV - from 20mm HS.404 case), 16x99 (US 16mm Vega exp)"
Cartridge #1 is a standard US .50 cal round for scale.
There were a variety of experimental guns to fire these cartridges.
The US wanted higher velocity for shorter flight times and increased hit probability (less lead needed). Please note that round #9 is a 20mm Hispano case necked to .50 cal and would need a Hispano cannon with new barrel to fire it (another reason these cartridges/rounds went nowhere, the size/weight of the guns) post war they necked the the 15.2x114 (US .60" T17) case out to 20mm and it became the round used in variety of US 20mm cannon including the Vulcan gun.
The US was NOT sitting back fat, dumb and happy with the .50 cal. They just went too far down the wrong road.
from Tony Williams website:
Cartridges 7,8, 9 and 10 from the left were ALL experimental cartridges worked on in WW II. "15.2x114 (US .60" T17), 12.7x114 (US .50/60"), 12.7x120 (US .50" HV - from 20mm HS.404 case), 16x99 (US 16mm Vega exp)"
Cartridge #1 is a standard US .50 cal round for scale.
There were a variety of experimental guns to fire these cartridges.
The US wanted higher velocity for shorter flight times and increased hit probability (less lead needed). Please note that round #9 is a 20mm Hispano case necked to .50 cal and would need a Hispano cannon with new barrel to fire it (another reason these cartridges/rounds went nowhere, the size/weight of the guns) post war they necked the the 15.2x114 (US .60" T17) case out to 20mm and it became the round used in variety of US 20mm cannon including the Vulcan gun.
The US was NOT sitting back fat, dumb and happy with the .50 cal. They just went too far down the wrong road.
They were happy enough with the performance of their .50 calibre machine guns throughout the war. They may have been spending a lot of money and resources on other heavy machine guns, were these intended for use in aircraft?
What they didn't have was the incentive to really push the development of cannon armament for their fighters (or other aircraft for that matter) as it was demonstrably not necessary given the operations being undertaken. When did the USAAF start to fit cannons as standard armament to its fighters? They might not have been sitting back "fat, dumb and happy" but as far as cannon for their aircraft go they were sitting back.
Cheers
Steve
- Thread starter
- #9
swampyankee
Chief Master Sergeant
- 3,954
- Jun 25, 2013
According to at least AG Williams, the USN's experience showed that the 20 mm was about 3 times as effective as the 0.5 in. Its shortcoming seemed to be the result of poor construction practices by the US ordnance industry, possibly because of a rather arbitrary division between "artillery" and "small arms."
The Oerlikon 20mm became a fairly standard ship mounted weapon early in the war for the US.
One thing US fighter might have had with the Hispano as opposed to the .50 cal - Less firing time to put rounds on target
As I'm thinking about it, more firing time but less damage per hit might be more helpful for a beginning to intermediate level pilot. Particularly if they are not going after heavy bombers.
And I'd definitely rather have 6 guns with 450 rounds each of .50 caliber than 4 hispanos of 60 rounds each like the British Spitfires if I'm shooting at a plane without self sealing fuel tanks.
Shortround6
Major General
They were pursuing the wrong path.
One reason the US .50 was as successful as it was is because of it's high velocity and well shaped bullet which meant A, a shorter time of flight than any other aircraft rounds except the Hispano, Soviet 12.7mm and German 15mm rounds. And only the 15mm was actually superior. This simplified the aiming problem. The US had a study that estimated that a 25% increase in velocity would result in 50% more hits (on average, and this is before the advent of the MK 14 sight and it's like) )(and would increase damage of a hit but that was secondary) and they went after this with a vengeance despite the cost in large heavy guns, burnt out barrels, weight of ammunition and broken guns.
You can't damage the enemy aircraft if you don't hit it and they were trying to increase the chances of hitting over the ability to do more damage with each hit.
This is of course in addition to a number of programs that were trying to increase the rate of fire on the standard Browning .50 cal. which took a number of years and only paid off in 1945, too late to have any real effect.
Part of the problem may have been to stringent requirements. Initial requirements called for not more than one breakage and 5 stoppages per 5,000 rounds fired.
High Standard handled the T22 series (7 different models)
Frigidaire handled the T25 series ( 4 models, the last of which was standardized as the M3)
High Standard also handled the T27 series (8 models)
Aberdeen Proving ground handled the T-26 and T-28.
Other programs came in at the end of the war and post war to boost rate of fire to 1500rpm.
.60 cal guns
Bendix modified a 20mm Hispano to take necked down ammuniton to 15mm.
Colt Started a program to copy the German MG 151 in 15mm except to use the American .60 cal cartridge (originally designed as an anti-tank rifle/MG round) , which required not only rechambering the barrel but lengthening the receiver, the feed way, bolt, cam tube and cover and other minor changes. This program shifted to Frigidaire and went through 5/6 models including both percussion and electric primed.
BTW max velocity was given as 3696fps (1120m/s).
There were a few other attempts to modify Hispano cannon to the smaller 15mm (.50 cal) round.
Work on .60 cal guns continued post war with 6 barreled Gatling guns being chambered for the round. Necking the case out to 20mm essentially resulted in the M61 Vulcan gun.
There were a rather bewildering array of 20mm gun projects under taken during the war. Including a number of attempts to sycronize the Hispano by various means (no explanations to why), modifying Hhispanos from gas (or partial gas) to recoil operated, modified MG 151s, lightened-high rate of fire versions ( finally standardized as the M3 20mm gun about the end of the war or shortly after) and a others.
And yes, these guns were intended for aircraft.
Try : http://photos.imageevent.com/badgerdog/generalstorage/georgemchinnthemachinegun/TheMGV3a.pdf
That was fine for the USAAF and USN engaging Japanese types and Luftwaffe fighters. It doesn't work when you have to engage heavily armoured bombers. The RAF was well aware that it's eight gun fighters were going to struggle with their rifle calibre machine guns in 1939, not long after they were introduced.
The arguments for eight guns had been based on calculations of the limited time on target the fighters were likely to have against high speed modern (in the mid 1930s) targets and rate of fire rather than weight of fire had been prioritised. The adoption by the Luftwaffe of relatively light armour started to show the flaw in the argument by 1940 and frantic and initially unsuccessful efforts were made to get a working 20mm cannon installation in the Spitfire. German bombers were returning to their continental bases with dozens or even hundreds of rifle calibre strikes. It wasn't long before a combination of two 20mm cannon and four .303 machine guns became the armament fitted to the vast majority of Spitfires built (some substituted two .50 calibre machine guns for the four .303s). Bigger fighters like the Typhoon and Tempest dispensed with machine guns altogether adopting a four cannon armament.
Both the Germans and Japanese, confronted with the heavily armoured US bombers adopted ever heavier cannon armament. Even 20mm canon was considered by both inadequate for the task they faced. Late war, some Luftwaffe fighters were abandoning their machine guns in favour of cannon armament, sometimes including a 30mm firing through the spinner. No one ever considered fitting machine guns of any calibre to the Me 262 or Do 335.
The USN caught up at the end of the war, the USAF (as it was by then)didn't really catch up until Korea.
Cheers
Steve
Until the US S/E fighters got the 6 HMG battery with plenty of ammo, RAF's fighters were flying with belt-fed Hispanos.
Edgar Brooks
Senior Airman
No Spitfire had only 4 cannon as its sole armament until the Mark 21, in late 1945; once the so-called "universal" wing became standard, from the Vc onward, it could carry 4 cannon + 4 .303" Brownings, though 2 + 4 was more normal.
Only the Ib, IIb, Vb VI cannon had just 60 rounds of 20mm ammunition, and, even then, they had 4 Brownings as back-up; in the universal set-up it was around 150 rounds per gun.
Also, if you have enemy bombers, with armour plate impervious to .303" or .5" bullets, roaming over your country, killing friends and family alike, you'd be praying for 20mm cannon to put a stop to it.
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Indeed. Being bombed is a wonderful incentive to devise better ways of destroying the bombers and killing their crews.
The British were aware of the problem well before the war as discussions about cannon armament start to become frequent and serious from about 1936 onwards. The Air Ministry instructed Supermarine to prepare a scheme to mount a 20mm Hispano cannon under each wing of the Spitfire in December 1938, nearly a year before the war started (for us).
It was Joseph Smith who decided that any cannon installation would have to be internal and though it was far from perfect the initial mock up was fitted to K9791 which was only the fifth production Spitfire.
Cheers
Steve
The British were aware of the problem well before the war as discussions about cannon armament start to become frequent and serious from about 1936 onwards. The Air Ministry instructed Supermarine to prepare a scheme to mount a 20mm Hispano cannon under each wing of the Spitfire in December 1938, nearly a year before the war started (for us).
It was Joseph Smith who decided that any cannon installation would have to be internal and though it was far from perfect the initial mock up was fitted to K9791 which was only the fifth production Spitfire.
Cheers
Steve
kool kitty89
Senior Master Sergeant
Wasn't the US Army Air Corps less fixated on the high velocity situation for much of the 1930s? The interest and experimentation with both the 37 mm M4 (or T9) autocannon (a rather conservative, long recoil operated gun, I believe) with its fairly low velocity and the 23 mm madsen with its intermediate velocity (albeit the use of belt feed was probably appealing compared to other cannons, and the limited magazine, short link belt, or endless-loop belts the T9) along with the .50 Browning only having a 2500 fps (762 m/s) muzzle velocity along with 750 grain projectiles up through its M1 iteration (the 1923/24 vintage ammunition up to the M1 specification of the 1930s was all in that range). Meanwhile, the .30-06 had been some 2700 fps in its 150 grain, flat based '06 form and dropped to approximately 2650 for the 173~175 grain M1 Ball, and then to 2750-2800 with the 150 gr flat-based M2 ball (plus a variety of AP, Incendiary, tracer, etc), and the M1 should've had the flattest trajectory over most combat ranges due to its superior ballistic coefficient and only slightly lower velocity than the other .30 cal loadings. (plus, I believe it was still heavily used in machine guns after the introduction of the M2, as the latter was, at least formally, done due to the M1 exceeding the range constraints of many US Army training ranges, with bullets penetrating backstops and posing a danger and the M2 dropped back to a bullet and loading mostly similar to the original '06 variant)
This also means the trajectory of the mixed .30 and .50 browning armaments in 1930s era fighters wouldn't have had matching trajectories, with the .30 being flatter shooting until the 1939 introduction of the M2 .50 loading with lighter 710 grain projectile and 2700 fps velocity. (OTOH it also would've meant the 720-ish m/s velocity of the 23 mm Madsen Cannon being tested at various points would've matched the older .50 cal loadings reasonably well, though obviously had poorer ballistics and noticeably more drop at longer ranges ... opposed to the 37mm M4, which would drop much more at all ranges on top of a very low rate of fire) Albeit, it's somewhat odd that the Oerlikon aircraft guns weren't in testing, or at least not nearly as prominently in that same period, particularly with the FFL's velocity quite close to the .50 cal loading in the 1930s. (750 m/s to 762 m/s)
There's also the extensive .90 caliber (23 mm) cannon project that the AAC/AAF undertook around the start of WWII and went through numerous designs of, which Tony Williams covered here: CAL90 (it appears they tried at least one oerlikon style API blowback system among those)
Though one consistently odd thing is that the well proven, reliable and cost-effective short-recoil mechanism of the Browning Machine gun wasn't scaled up beyond .50 caliber, and any efforts or proposals to do so were rejected by the Army (not sure if such were ever presented to the Navy). It seems like a potentially more foolproof design, and one that could've been adapted to other calibers/chamberings being tested (like the 20 mm hispano and 23 mm Madsen, or potentially the somewhat higher velocity 20 mm Madsen, which would also be simpler/cheaper to convert between the 23 and 20 mm) along with the .60 cal (15.2 mm) anti-tank cartridge used in the MG-151 derivative. Once the Hispano was adopted, it seems like a Browning derived cannon would be a smart fallback plan on top of potentially avoiding patent/license/royaltee fees and having a potentially lighter, better performing gun with more maintenance commonality with existing equipment, plus the definite ability to be synchronized with a propeller, which certainly seemed to at least be a minor interest. (and would've been at least significant for employing on the P-39 and P-63 in place of the .50 cal nose guns and potentially as the nose guns on the early P-51 models) The short-recoil action also may have tolerated the apparent problems with reliability caused by differing standards of manufacturing 'artillery' equipment. (I'm not sure this was actually the main problem or persistent problem with the Hispano given the M3 continued to have problems, as did the Navy's Mk.12, and the M3 worked around the most common problem of lightly struck primers by implementing an electrical recocking mechanism, though had they adopted electric priming like the British later did, it probably would've addressed most reliability issues: fixation on directly copying the French HS.404 pattern, followed by US-specific changes and refusal to adopt successful British modifications seems to have been a considerable problem, but if it was an issue of Not-Invented-Here or some such, the lack of use of Colt/Browning machine gun mechanisms for cannons is even stranger)
Also, given the extremely conservative 150 RPM requirement for the .90 cal gun, it seems odd that a derivative of the 37 mm M4 cannon wasn't trialed, given necking down the existing cartridge to 23 mm seems like it would've easily met or exceeded the velocity requirements and getting the existing gun's 96 kg weight down to the 57 kg requirement and ammo capacity to 50 rounds rather than the 30 round loop belt seem within the realm of possibility or close enough that a reliable, but somewhat heavier cannon resulting from such would've been seriously considered.
Hell, for that matter, it might have made some sense to scale down the long-recoil action of the M4 cannon to fit the 20 mm Hispano cartridge (at presumably roughly similar weight to the Hispano cannon, and probably lower rate of fire, but much better than 120 RPM) given it seemed to cope well with the existing manufacturing standards and was quite reliable aside from issues resulted to clogged ejection chutes on early P-39 models. (not a fault of the gun) Albeit, I'm fairly certain it would've been more costly and poorer performing than a Browning Machine Gun derivative. (long recoil mechanisms are fairly costly and complex to manufacture compared to short recoil, particularly assuming this was based on the same old Browning long recoil action that the Auto 5 shotgun, Remington model 8, and french Cheauchat were based on) The Madsen cannons were also a form of long recoil, derived from the Madsen machine gun, and were also rather expensive to manufacture (though known to be quite robust and reliable) so it's at least somewhat understandable on that end as to why there wasn't a major effort to license-produce that weapon pre-war. (though it must have been seen as a more potent and practical design than the 37 mm M4 and certainly should've been a better match to the P-39 and early P-38 models than the 37 mm gun, particularly examples using the 15 round belt and especially as it was belt-fed, unlike most or all 20mm cannons available at the time, including the Madsen 20 mm itself, which was magazine-fed and sold primarily for the AA role, though I imagine the 23 mm aircraft gun would've been easily adapted back to the 20 mm cartridge)
It's also interesting that the Soviet VYa-23 cannon would've been almost perfect for what the Army was interested in, though its reliability might have been suspect the velocity was high, weight was relatively low, and rate of fire was quite good.
The Army also probably would've been somewhat interested in a gun with the qualities of the German MG c/30L once they hit their high-velocity fixation, and it was in the same weight class as the Hispano and probably would've tolerated 'artillery' manufacturing standards as well. (though the rate of fire was in the vein of the Madsen at 300-350 RPM and magazine or drum fed and its ground-based Flak 38 counterpart was developed to up to 450 RPM ... the overall weight/performance seems arguably superior to the Oerlikon FFS or certainly the older type S or related French HS.9, which I think is the Oerlikon gun the British experimented with as a fighter armament as well)
I also find it somewhat odd that, after all that interest in 23 mm, the Army, or USAF would settle on necking out the .60 cal cartridge to 20mm rather than 23 mm. (quite possibly with the same high velocity and just a moderately lower rate of fire than the M39 ... or just a moderately heavier gun) It seems like a more flexible and sensible mix of characteristics than 20 mm or the lower velocity 30 mm cartridges being adopted in Europe, or the high velocity 30 mm cartridges being trialed/offered with much heavier guns. (Oerlikon had a few designs offering such in the late 40s and early 50s, and Sweden eventually adopted the KCA for the Saab 37 Viggen)
This also means the trajectory of the mixed .30 and .50 browning armaments in 1930s era fighters wouldn't have had matching trajectories, with the .30 being flatter shooting until the 1939 introduction of the M2 .50 loading with lighter 710 grain projectile and 2700 fps velocity. (OTOH it also would've meant the 720-ish m/s velocity of the 23 mm Madsen Cannon being tested at various points would've matched the older .50 cal loadings reasonably well, though obviously had poorer ballistics and noticeably more drop at longer ranges ... opposed to the 37mm M4, which would drop much more at all ranges on top of a very low rate of fire) Albeit, it's somewhat odd that the Oerlikon aircraft guns weren't in testing, or at least not nearly as prominently in that same period, particularly with the FFL's velocity quite close to the .50 cal loading in the 1930s. (750 m/s to 762 m/s)
There's also the extensive .90 caliber (23 mm) cannon project that the AAC/AAF undertook around the start of WWII and went through numerous designs of, which Tony Williams covered here: CAL90 (it appears they tried at least one oerlikon style API blowback system among those)
Though one consistently odd thing is that the well proven, reliable and cost-effective short-recoil mechanism of the Browning Machine gun wasn't scaled up beyond .50 caliber, and any efforts or proposals to do so were rejected by the Army (not sure if such were ever presented to the Navy). It seems like a potentially more foolproof design, and one that could've been adapted to other calibers/chamberings being tested (like the 20 mm hispano and 23 mm Madsen, or potentially the somewhat higher velocity 20 mm Madsen, which would also be simpler/cheaper to convert between the 23 and 20 mm) along with the .60 cal (15.2 mm) anti-tank cartridge used in the MG-151 derivative. Once the Hispano was adopted, it seems like a Browning derived cannon would be a smart fallback plan on top of potentially avoiding patent/license/royaltee fees and having a potentially lighter, better performing gun with more maintenance commonality with existing equipment, plus the definite ability to be synchronized with a propeller, which certainly seemed to at least be a minor interest. (and would've been at least significant for employing on the P-39 and P-63 in place of the .50 cal nose guns and potentially as the nose guns on the early P-51 models) The short-recoil action also may have tolerated the apparent problems with reliability caused by differing standards of manufacturing 'artillery' equipment. (I'm not sure this was actually the main problem or persistent problem with the Hispano given the M3 continued to have problems, as did the Navy's Mk.12, and the M3 worked around the most common problem of lightly struck primers by implementing an electrical recocking mechanism, though had they adopted electric priming like the British later did, it probably would've addressed most reliability issues: fixation on directly copying the French HS.404 pattern, followed by US-specific changes and refusal to adopt successful British modifications seems to have been a considerable problem, but if it was an issue of Not-Invented-Here or some such, the lack of use of Colt/Browning machine gun mechanisms for cannons is even stranger)
Also, given the extremely conservative 150 RPM requirement for the .90 cal gun, it seems odd that a derivative of the 37 mm M4 cannon wasn't trialed, given necking down the existing cartridge to 23 mm seems like it would've easily met or exceeded the velocity requirements and getting the existing gun's 96 kg weight down to the 57 kg requirement and ammo capacity to 50 rounds rather than the 30 round loop belt seem within the realm of possibility or close enough that a reliable, but somewhat heavier cannon resulting from such would've been seriously considered.
Hell, for that matter, it might have made some sense to scale down the long-recoil action of the M4 cannon to fit the 20 mm Hispano cartridge (at presumably roughly similar weight to the Hispano cannon, and probably lower rate of fire, but much better than 120 RPM) given it seemed to cope well with the existing manufacturing standards and was quite reliable aside from issues resulted to clogged ejection chutes on early P-39 models. (not a fault of the gun) Albeit, I'm fairly certain it would've been more costly and poorer performing than a Browning Machine Gun derivative. (long recoil mechanisms are fairly costly and complex to manufacture compared to short recoil, particularly assuming this was based on the same old Browning long recoil action that the Auto 5 shotgun, Remington model 8, and french Cheauchat were based on) The Madsen cannons were also a form of long recoil, derived from the Madsen machine gun, and were also rather expensive to manufacture (though known to be quite robust and reliable) so it's at least somewhat understandable on that end as to why there wasn't a major effort to license-produce that weapon pre-war. (though it must have been seen as a more potent and practical design than the 37 mm M4 and certainly should've been a better match to the P-39 and early P-38 models than the 37 mm gun, particularly examples using the 15 round belt and especially as it was belt-fed, unlike most or all 20mm cannons available at the time, including the Madsen 20 mm itself, which was magazine-fed and sold primarily for the AA role, though I imagine the 23 mm aircraft gun would've been easily adapted back to the 20 mm cartridge)
It's also interesting that the Soviet VYa-23 cannon would've been almost perfect for what the Army was interested in, though its reliability might have been suspect the velocity was high, weight was relatively low, and rate of fire was quite good.
The Army also probably would've been somewhat interested in a gun with the qualities of the German MG c/30L once they hit their high-velocity fixation, and it was in the same weight class as the Hispano and probably would've tolerated 'artillery' manufacturing standards as well. (though the rate of fire was in the vein of the Madsen at 300-350 RPM and magazine or drum fed and its ground-based Flak 38 counterpart was developed to up to 450 RPM ... the overall weight/performance seems arguably superior to the Oerlikon FFS or certainly the older type S or related French HS.9, which I think is the Oerlikon gun the British experimented with as a fighter armament as well)
I also find it somewhat odd that, after all that interest in 23 mm, the Army, or USAF would settle on necking out the .60 cal cartridge to 20mm rather than 23 mm. (quite possibly with the same high velocity and just a moderately lower rate of fire than the M39 ... or just a moderately heavier gun) It seems like a more flexible and sensible mix of characteristics than 20 mm or the lower velocity 30 mm cartridges being adopted in Europe, or the high velocity 30 mm cartridges being trialed/offered with much heavier guns. (Oerlikon had a few designs offering such in the late 40s and early 50s, and Sweden eventually adopted the KCA for the Saab 37 Viggen)
Very interesting thread!
kool kitty89
Senior Master Sergeant
I mis-spoke above about the 23 mm cannon US Army project: one of the initial prototypes was a Browning Long Recoil weapon with similar roots to the M4/T9 37 mm weapon, and it was rejected mostly due to its weight still being above the specification. However, it used a different cartridge case and feed system (apparently a 50 round pan magazine) than the T9 and wouldn't be a simple barrel swap (and possibly recoil spring change), so the question there would be: as an interim measure, why wasn't a more conservative modification to an existing production weapon considered? (even with poor rate of fire and not much weight reduction over the M4/T9, it would have better ballistics ... though if necking down the existing 37 mm cartridge and using the same receiver/breech/recoil mechanism, 23 mm would probably be overkill, and something in the 25-30 mm range would have gotten ballistics more in the Oerlikon S/FF S, Hispano, or .50 BMG range while having better shell capacity)
I'd think a weapon scaled down further than the M4 would be more realistic for a cartridge closer to the specification the Army had in mind. (it still would've been a relatively mediocre performer, but smaller and lighter at least, possibly more in the realm of the 23 mm Madsen or Hispano)
Additionally, the Madsen guns (of all calibers) were definitely turned down primarily due to cost/complexity of manufacture specific to their mechanics. (the Browning Long Recoil system was a good deal more economical, if still more complex/costly and much slower firing than the Browning Short Recoil system of the M1919 and derivatives)
https://www.forgottenweapons.com/wp-content/uploads/2015/05/Aberdeen test of Madsen 7mm and 20mm.pdf
see pg 12 for concern over manufacturing complexity of the Madsen mechanism (in particular, they cited the use of many complex curved surfaces being used, requiring complex, skilled machining operations)
It's possible that some or most of those curved parts could've been made cost-effective as stamped or possibly forged parts, but I'm not sure when that became a viable option in US industry and it doesn't seem to have occurred to anyone at the time.
OTOH, if the ballistics of the 23 mm Madsen gun were considered at least acceptable, or if the US Navy had taken a greater interest in such (in spite of somewhat lower velocity), they also might have considered adapting the 20x110mm Hispano or 20x110RB cartridge of the Oerlikon Type S and FF S to 23 mm in a similar manner to the Madsen cartridge.
The Oerlikon gun might have been more attractive in this case as it would take a bottlenecked case and make it straight or nearly straight walled and thus get a fair bit more efficiency out of the same spring, receiver, and bolt of the existing FF S design: blowback and API blowback weapons favor straight-walled case as bolt-thrust has a nearly linear relationship between the ratio of the area of the base of the projectile and the base of the cartridge, which is also why the other 20 mm Oerlikon guns and German and Japanese 30 mm API Blowback weapons also use straight-walled cases. The chamber would still need to be strong enough to withstand the pressures involved, but that would be part of the barrel swap, and might otherwise mean using the FF S unchanged (albeit developing a belt-feed system for it would be high on the priorities and working on modifications to improve rate of fire), but I'd think the overall weight savings and performance compared to adapting the Hispano to 23 mm, even before they realized the reliability issues, would have been major considerations for the US Navy, plus the potential to switch between 20 and 23 mm configurations relatively easily and use the same ammunition as the Type S anti-aircraft guns being adopted on various warships. (this latter bit might also be a reason it would be a more likely consideration than the lighter, shorter, lower recoiling FF L cannon, which had similar muzzle velocity and roughly similar ballistics to the pre-war .50 cal loadings) Had they opted to copy the 174 gram Madsen projectiles and employ them in a modified FF S cannon, the muzzle velocity should've been around 770 m/s and sectional density somewhat higher than the 122 or 123 gram HE/HE-I/HE-T projectiles of the type S. (if they dropped to a rather light 162-ish gram projectile, it should've allowed around 820 m/s with similar sectional density to the 20 mm, but unless a relatively long, streamlined, but light projectile was developed, it would have a poorer BC and poorer ballistics than the 20 mm; a relatively short, light 23 mm shell with a fairly long, streamlined nose cap should have been easy enough to engineer if they were after higher velocity and good ballistics, plus ballistic shaping should be easier than with the typical 20 mm projectiles)
Blowback Gun Design note the section on bottlenecked case designs
The Hispano, OTOH would either have to reduce the power of its ammunition significantly to use the same mechanism, or strengthen the mechanism and add weight to cope with a more powerful cartridge. (the Madsen 20 mm cannon started out with an already somewhat more potent cartridge, so converting to 20 mm and adding a belt-feed mechanism didn't impact weight or rate of fire much)
Also, not related to US developments, but the pre-war, high power 20 mm autocannons of the IJA and Germany, both derived from anti-tank and AA guns, while rather mediocre and overweight to be efficient fighter weapons, used ammunition/cartridges that could/should have adapted well to 23-25 mm variants via simply necking them up. The MG c/30L and Ho-3 respectively firing the Swiss 20x138mmB and Japanese 20x125mm cartridges. The ground-based Flak-38 counterparts to the MG c/30L managed at least 450 RPM in later developments, so it seems reasonable that could've been maintained, and necking it out to 23 or 25 mm (possibly using Danish 23 mm or French 25 mm Madsen shell manufacturing to supplement such) should have been straightforward for producing a pretty potent early-war anti-bomber weapon, and hypothetically one that could also be synchronised and mounted in Fw 190 wing roots. (with a new cartridge, it should also have been possible to adapt it to the same 30 mm shells as the MK 101 was using for a low/medium velocity gun closer to MG FF ballistics and probably moderately higher velocity, but slower firing and somewhat heavier than the MK 108, but also available much earlier in the war and vastly lighter and faster firing than the MK 101)
The Ho 3 likewise should've used a necked-out cartridge relatively easily and possibly a 30 mm cartridge. (using the fairly light 253 g 30 mm shell of the later Ho 155 gun, velocity would've probably been closer to 600 m/s or slightly less, allowing for similar or slightly lower muzzle energy than the 20x125 cartridge and gun weight and rate of fire would also likely be similar to the Ho 155, but it should've been a much earlier war weapon) OTOH, 23-25 mm would seem a more obvious route and probably fit the existing cartridge case better. (I had a hard time finding the dimensions of the 20x125 Japanese cartridge ... the Swiss 20x138B has measurements listed online and wouldn't practically allow more than 25 mm)
Additionally, the British, during their trials of FN Browning aircraft heavy machine guns could/should have considered those chambered for the 13.2 mm Hotchkiss gun, or the 13.2x99mm derivative (basically .50 BMG necked up to take the 13.2 mm Hotchkiss projectiles) as they were loaded to higher velocities with better armor piercing characteristics than either American or British (Kynoch Export loads were slightly heavier and hotter than pre-war American .50 loads) cartridges and on top of the high rate of fire (1050 RPM in British Trials) those guns should've been seriously considered to fit the sorts of qualities the 20 mm Hispano was being developed for, but with the advantages of being easier to adapt to existing fighters as wing or synchronized nose/fuselage mounts. (ie Gloster Gladiators could even be re-armed with them, and the high ROF of FN guns should have made synchronized mounts much more reasonable than the 500-550 RPM typical of American M2 Brownings) The 13.2 mm projectiles also should've had marginally better incendiary capacity for what that was worth at the time, though would become more important once improved incendiary and API developments were introduced.
A lighter Browning derivative developed around the .50 Vickers cartridge should also have been possible, but would've been less ready-made and have thus required greater interest (plus further development of the Vickers .50 gun was more appealing for a time as it was initially more reliable and lighter than the .50 Browning in 1920s testing) but that cartridge had mediocre armor penetration and thus lack of interest. (I'm not sure why lighter, higher velocity AP bullet loads weren't developed, both for aircraft and AA use, as the existing ball and AP loads in the 2500-2540 fps range were rather mediocre, and while already light for .50 cal weapons at 580 grams, should still have seen similar advantages for lightening + higher velocity and higher muzzle energy on top of much better armor penetration within effective ranges ... plus a good deal of production had used flat-based projectiles, so lighter ones with better ballistic shapes could have had better all-around ballistics on top of good armor penetration, relatively light recoil, and relatively light gun weight) It's also noteworthy that the bullet shapes of .5 Vickers cartridges were already superior to those of their export derivatives used later by the Italians and Japanese and the 1920s era Vickers .5 inch aircraft gun was 24 kg compared to the 29 kg (and similar 700 RPM rate of fire) of the Breda-SAFAT heavy machine gun used by the Italians, which was perhaps the most mediocre one of the war. (the WWI vintage Vickers gun had also been prone to jams/stoppages in aircraft use, but I'm not sure how that translated to later marks or the .50 cal guns and their switch to disintegrating metal link belts, or the comparable .303 Vickers marks that had been lightened and sped up to 900 RPM that lost out to the still lighter and better performing .303 Browning, though the Japanese Army and Navy used similar 900 RPM vickers developments)
I imagine the license to manufacture .303 Browning machine guns would've been competing with any hypothetical interest in a 13.2 mm weapon. (and I'd think either would go through FN, since they generally dealt with Colt/browning gun licenses in Europe)
I'd think a weapon scaled down further than the M4 would be more realistic for a cartridge closer to the specification the Army had in mind. (it still would've been a relatively mediocre performer, but smaller and lighter at least, possibly more in the realm of the 23 mm Madsen or Hispano)
Additionally, the Madsen guns (of all calibers) were definitely turned down primarily due to cost/complexity of manufacture specific to their mechanics. (the Browning Long Recoil system was a good deal more economical, if still more complex/costly and much slower firing than the Browning Short Recoil system of the M1919 and derivatives)
https://www.forgottenweapons.com/wp-content/uploads/2015/05/Aberdeen test of Madsen 7mm and 20mm.pdf
see pg 12 for concern over manufacturing complexity of the Madsen mechanism (in particular, they cited the use of many complex curved surfaces being used, requiring complex, skilled machining operations)
It's possible that some or most of those curved parts could've been made cost-effective as stamped or possibly forged parts, but I'm not sure when that became a viable option in US industry and it doesn't seem to have occurred to anyone at the time.
OTOH, if the ballistics of the 23 mm Madsen gun were considered at least acceptable, or if the US Navy had taken a greater interest in such (in spite of somewhat lower velocity), they also might have considered adapting the 20x110mm Hispano or 20x110RB cartridge of the Oerlikon Type S and FF S to 23 mm in a similar manner to the Madsen cartridge.
The Oerlikon gun might have been more attractive in this case as it would take a bottlenecked case and make it straight or nearly straight walled and thus get a fair bit more efficiency out of the same spring, receiver, and bolt of the existing FF S design: blowback and API blowback weapons favor straight-walled case as bolt-thrust has a nearly linear relationship between the ratio of the area of the base of the projectile and the base of the cartridge, which is also why the other 20 mm Oerlikon guns and German and Japanese 30 mm API Blowback weapons also use straight-walled cases. The chamber would still need to be strong enough to withstand the pressures involved, but that would be part of the barrel swap, and might otherwise mean using the FF S unchanged (albeit developing a belt-feed system for it would be high on the priorities and working on modifications to improve rate of fire), but I'd think the overall weight savings and performance compared to adapting the Hispano to 23 mm, even before they realized the reliability issues, would have been major considerations for the US Navy, plus the potential to switch between 20 and 23 mm configurations relatively easily and use the same ammunition as the Type S anti-aircraft guns being adopted on various warships. (this latter bit might also be a reason it would be a more likely consideration than the lighter, shorter, lower recoiling FF L cannon, which had similar muzzle velocity and roughly similar ballistics to the pre-war .50 cal loadings) Had they opted to copy the 174 gram Madsen projectiles and employ them in a modified FF S cannon, the muzzle velocity should've been around 770 m/s and sectional density somewhat higher than the 122 or 123 gram HE/HE-I/HE-T projectiles of the type S. (if they dropped to a rather light 162-ish gram projectile, it should've allowed around 820 m/s with similar sectional density to the 20 mm, but unless a relatively long, streamlined, but light projectile was developed, it would have a poorer BC and poorer ballistics than the 20 mm; a relatively short, light 23 mm shell with a fairly long, streamlined nose cap should have been easy enough to engineer if they were after higher velocity and good ballistics, plus ballistic shaping should be easier than with the typical 20 mm projectiles)
Blowback Gun Design note the section on bottlenecked case designs
The Hispano, OTOH would either have to reduce the power of its ammunition significantly to use the same mechanism, or strengthen the mechanism and add weight to cope with a more powerful cartridge. (the Madsen 20 mm cannon started out with an already somewhat more potent cartridge, so converting to 20 mm and adding a belt-feed mechanism didn't impact weight or rate of fire much)
Also, not related to US developments, but the pre-war, high power 20 mm autocannons of the IJA and Germany, both derived from anti-tank and AA guns, while rather mediocre and overweight to be efficient fighter weapons, used ammunition/cartridges that could/should have adapted well to 23-25 mm variants via simply necking them up. The MG c/30L and Ho-3 respectively firing the Swiss 20x138mmB and Japanese 20x125mm cartridges. The ground-based Flak-38 counterparts to the MG c/30L managed at least 450 RPM in later developments, so it seems reasonable that could've been maintained, and necking it out to 23 or 25 mm (possibly using Danish 23 mm or French 25 mm Madsen shell manufacturing to supplement such) should have been straightforward for producing a pretty potent early-war anti-bomber weapon, and hypothetically one that could also be synchronised and mounted in Fw 190 wing roots. (with a new cartridge, it should also have been possible to adapt it to the same 30 mm shells as the MK 101 was using for a low/medium velocity gun closer to MG FF ballistics and probably moderately higher velocity, but slower firing and somewhat heavier than the MK 108, but also available much earlier in the war and vastly lighter and faster firing than the MK 101)
The Ho 3 likewise should've used a necked-out cartridge relatively easily and possibly a 30 mm cartridge. (using the fairly light 253 g 30 mm shell of the later Ho 155 gun, velocity would've probably been closer to 600 m/s or slightly less, allowing for similar or slightly lower muzzle energy than the 20x125 cartridge and gun weight and rate of fire would also likely be similar to the Ho 155, but it should've been a much earlier war weapon) OTOH, 23-25 mm would seem a more obvious route and probably fit the existing cartridge case better. (I had a hard time finding the dimensions of the 20x125 Japanese cartridge ... the Swiss 20x138B has measurements listed online and wouldn't practically allow more than 25 mm)
Additionally, the British, during their trials of FN Browning aircraft heavy machine guns could/should have considered those chambered for the 13.2 mm Hotchkiss gun, or the 13.2x99mm derivative (basically .50 BMG necked up to take the 13.2 mm Hotchkiss projectiles) as they were loaded to higher velocities with better armor piercing characteristics than either American or British (Kynoch Export loads were slightly heavier and hotter than pre-war American .50 loads) cartridges and on top of the high rate of fire (1050 RPM in British Trials) those guns should've been seriously considered to fit the sorts of qualities the 20 mm Hispano was being developed for, but with the advantages of being easier to adapt to existing fighters as wing or synchronized nose/fuselage mounts. (ie Gloster Gladiators could even be re-armed with them, and the high ROF of FN guns should have made synchronized mounts much more reasonable than the 500-550 RPM typical of American M2 Brownings) The 13.2 mm projectiles also should've had marginally better incendiary capacity for what that was worth at the time, though would become more important once improved incendiary and API developments were introduced.
A lighter Browning derivative developed around the .50 Vickers cartridge should also have been possible, but would've been less ready-made and have thus required greater interest (plus further development of the Vickers .50 gun was more appealing for a time as it was initially more reliable and lighter than the .50 Browning in 1920s testing) but that cartridge had mediocre armor penetration and thus lack of interest. (I'm not sure why lighter, higher velocity AP bullet loads weren't developed, both for aircraft and AA use, as the existing ball and AP loads in the 2500-2540 fps range were rather mediocre, and while already light for .50 cal weapons at 580 grams, should still have seen similar advantages for lightening + higher velocity and higher muzzle energy on top of much better armor penetration within effective ranges ... plus a good deal of production had used flat-based projectiles, so lighter ones with better ballistic shapes could have had better all-around ballistics on top of good armor penetration, relatively light recoil, and relatively light gun weight) It's also noteworthy that the bullet shapes of .5 Vickers cartridges were already superior to those of their export derivatives used later by the Italians and Japanese and the 1920s era Vickers .5 inch aircraft gun was 24 kg compared to the 29 kg (and similar 700 RPM rate of fire) of the Breda-SAFAT heavy machine gun used by the Italians, which was perhaps the most mediocre one of the war. (the WWI vintage Vickers gun had also been prone to jams/stoppages in aircraft use, but I'm not sure how that translated to later marks or the .50 cal guns and their switch to disintegrating metal link belts, or the comparable .303 Vickers marks that had been lightened and sped up to 900 RPM that lost out to the still lighter and better performing .303 Browning, though the Japanese Army and Navy used similar 900 RPM vickers developments)
I imagine the license to manufacture .303 Browning machine guns would've been competing with any hypothetical interest in a 13.2 mm weapon. (and I'd think either would go through FN, since they generally dealt with Colt/browning gun licenses in Europe)
Glider
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The USN were always keen to field the 20mms and the Hellcat F6F5 was designed to carry 2 x 20 and 4 x 0.5 would certainly have had them.
The first Mustangs had 4 x 20 and I see no reason why they would have changed. I am certain the P47 would have had 4 x 20mm.
The first Mustangs had 4 x 20 and I see no reason why they would have changed. I am certain the P47 would have had 4 x 20mm.
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