I've added a new article to my website, this one just for fun: a look at an alternative choice of 1930s tank gun for the British Army.

Right, be prepared to be teleported to England circa 1933. Go see Winston with your plan while you are at it take the design for the Cromwell with you. We can also send you back to 1940, take a Hi-Power shoot the members of the tank board, leave plans for the Comet and 17pdr.

I suspect you might also have to shoot a few members of the Royal Artillery who believed that anything with HE belonged to them.

Interesting thoughts, what about a vickers light tank chassis with the 18pdr as a assault gun?

While he's at it, have him shoot the nuts that fomented the British Leyland fiasco.

Which particular British Leyland fiasco are you referring to? I'm curious.

I know there was one with jet engine development, which basically cost two years in getting engines into production and was eventually solved by handing things over to Rolls Royce. Was there one to do with tanks?

The one where entire industries were forced to merge under one company.

Been there, done that: see The Foresight War


Assault guns by their nature have to be able to resist direct fire at short ranges - so need thick armour. I can't see a light tank coping with the weight of that as well as an 18 pdr gun. Now fitting one in a limited-traverse mounting on a Matilda II chassis sounds doable.

It didn't stop the Germans from doing it, they built a SPG on captured vickers as I recall. They would have been useful in support of the Matilda I's as would a 2pdr on the carrier.

Just read the first chapter of your book, will have to find it!

Question for Tony W:

Having read your interesting Alternative,
my question is,
in the 4th paragraph, you suggest that "The barrel of our new gun would be restricted to 43 calibres because that's all that the available machinery could cope with at that time,..."

Not really ever having delved into the foundry and metalworking technologies of England of that period myself,
what exactly was the restriction?
Was it a length vs bore diameter issue (couldn't fabricate any gun barrels greater than ~43 cals of any bore diameter),

or was it an overall actual length issue (57x43=2451mm, or ~96&1/2 inches, roughly 8 feet, and no small/medium caliber lathes in that diameter could achieve greater barrel lengths) ?

If the latter, would/could there have been much desire to produce a longer barrel 2-pdr, akin to how the Germans first had an L42, then went to an L60, 50mm gun?
Would a theoretical ~60 cal (just under 8 feet of barrel) 2pdr gun have had any worthwhile merit, other than the possibility of considerably better AP performance than the stock 2pdr?
Yeah, much more difficult to manhandle a towed gun with an 8 foot barrel, but would've worked well enough for tank turrets.

Myself never really seeing true merit in the squeezebore principle since it hampers/limits HE performance,
and also realizing the great deficiency of the 2pdr was the low priority in giving it a useful HE support round early on,
would it have been worth it to pursue a longer, higher-performance 2pdr gun (40mm) solely on the grounds of improved AP,
or was it just better that so many were left behind during the evac from mainland Europe and further encourage the development of the 6pdr?

Must admit, I do like you idea concerning the 57mm cartridge.

Along the lines as what I suggested of the 2pdr, would it have served the British worthwhile to have further developed the 3pdr gun (47mm?) into a longer barrel weapon with a stronger AP performance, in addition to having superior HE potential over any 40mm ammunition?
Was there an alternative 47mm casing/cartridge in anyone's inventory that could've proven feasible to improve the 3pdr just so?
Or any 40mm case (other than Bofors of the day) that would've allowed a better-performing 2pdr without deducting too much from ammo capacity due to a bigger case?

If the 3pdr had developed further, would there have been any sense at all in even drafting up the 2pdr?

Much obliged.

Seeing as a Valentine chassis proved suitable enough to mount a 25pdr and create a monstrosity (even if effective) called Bishop,
maybe doing an 18pdr first wouldn't have been a bad idea: if the 18pdr would've come about on a Valentine chassis (or dare I suggest Covenanter? or early Crusader?) looking like the boxy and trav/elev limited Bishop, perhaps when it came time to develop a 25pdr SP gun, the British might've come up with a better design than Bishop to mount it...?

I'm not certain, but I know it wasn't possible to make 6 pdr guns with the planned L/50 barrels until later in the war, the first Marks being limited to L/43 (the US-made version having 50 cal from the start). I presume that the bigger lathes were being used to make more urgently-required big guns.

This was actually done, although it never saw service. See THIS article on my website.


There was a more powerful Hotchkiss 3 pdr naval gun but the ammo was longer than the 6 pdr's. The French also had some useful 47mms. However, I would still go with the 6 pdr to get double the HE performance.

.
No - the Bofors round was slightly more powerful (in fact, I have that adopted for the tank/antitank gun in The Foresight War), but there weren't any others. The French had some powerful 37mm rounds, though.

Birch gun anyone?

in 1928, 18pdr on a Vickers medium chassis.


In terms of British tanks in late 1930s, what about the best British tank of the 1930s that never saw service with the British Army, the Vickers 6 tonner? Given an uprated 3pdr gun you have a T-26.

One should not confuse a self-propelled artillery peice with an assault gun.

The SP-gun uses it's tracks (wheels?) to move from one firing position to another much more quickly than a towed gun could. The tracked versions also can negotiate terrain or weather conditions better than a towed gun and so keep up with the armour for support use. If your tanks can cover 60-100 kilometers ina day but your trucks/towed guns get stuck/bogged and only cover 30-50 kilometers your armour will soon be in trouble.
While some SP-guns were used on occasion as assault guns it was not the norm and could only result in high causulties if the oposition had ANY sort of anti-tank weapons. Loosing the divisions artillery assests in a direct assault doesn't win many battles or promotions.
British once used towed 5.5in guns in an assault on a defended position in direct fire mode at ranges under 1000yds but that is not how they were normally used.

I have read that the 3pdr ammunition still used black powder as a propellent. I don't know if that is true but the basic design and performance certainly date from that time. A hot rodded 3pdr round is certainly possiable given a stronger gun to hold it.

A consideration of gun/projectile design at the time was price. The 2pdr shot was pretty cheap stuff. A chunk of steel of an apropreate alloy, suitably heat treated and fitted with a tracer element and driving band. Changing to APHE like German 37mm/50mm ment more machining of the rear end, filling with HE, and fitting a suitable fuse. Many books claim there is a velocity threshold above which simple shot shatters when striking armour. The threshold seems to vary according to which book you read but that may also be the result of different alloys, different heat treat and different armour targets (face hardened armour may shatter shot at a lower velocity). Putting a piercing cap on the shot extends the striking velocity upwards by several hundred feet per second after which you need a harder/tougher material like tungsten carbide to withstand the impact. Piercing caps are rather poor from a streamlining point of view and so need to be covered by a ballistic cap. Fitting these caps means more parts/ machining operations and inspections to ensure correct assembly and so the price per round goes up and/or the production per day goes down.
At any given velocity the piercing ability of a shot is in proportion to it's caliber. Assuming you have a striking velocity that gives a result 1.3 times the caliber you would have a 40mm gun getting 52mm of penetration, a 47mm getting 61mm, a 57mm=74mm, a 75mm= 97.5mm and an 88mm=114mm. This does help explain why gun designers went to bigger guns, they simply had to scale up what they knew already worked (and given that most gun designers/factories also worked on naval guns they might have been scaling down) rather than work out problems with higher than normal velocity guns. The larger calibers held their velocity better than the smaller ones so that for a given muzzle velocity the striking velocity shifted more in favor of the larger guns the further away the target was.

Once the shooting really starts the purse strings loosen up quite a bit and if combat results show that present equipment isn't quite up to par with the enemy (or what is thought the enemy might come up with next) then many avenues got explored--hyper velocity guns, special ammo and more.

For an alternative history I guess it depend on how much money the treasury will come up with and when. While a 6pdr round of ammo won't cost 3 times that of a 2pdr there is going to be a significant increase in the cost per round which might affect peacetime thinking;-)

Bofors ammo was actually used in tanks, Hungarian Turan series.

My raising of the Birch gun was in the context of it being suggested that the Germans adopted various captured light armoured vehicles to carry German 105 howitzers and of course 75mm PaK guns) and 150mm infantry support weapons. None of theses were assault guns as such, but it would be fair to say that the various 150mm infantry howitzers would have been used both in indirect and direct fire mode. The Birch gun, whilst not an assault gun, would still have been capable, in the 1930s context of 'stand off' direct fire. Not closing within effective range of whatever enemy AT guns existed (in 1928 there would have been damned few, same in 1935) and using direct HE to pound the enemy's 'AT Gun Line'.

It should also be remembered that until the late 1930s few guns had a traversing mechanism capable of dealing with the movement of a mechanised opposition. It was this requirement that lead to the 25pdr having a turntable, and a foresighted requirement that was too.

Even in 1939 Anti-Tank engagements were intended to take place at less than 500 yards, The Birch gun would have been able to exceed this range by a handsome margin.

By 1941/1942 the Birch gun would have been seriously obsolete. But by that time the British may have realised the need for a proper assault gun. It should also be remember that later in the war the Harry Hopkins was proposed as an assault gun / airborne support weapon. If this had gone into service in 1941 it would have been very useful. By 1944 it was vulnerable and out of date.

The British also had a range of mountain guns (actually howitzers) that could have been usefully mounted even on Vickers light chassis in 1938 Could have provided good support to those lights that went into action in France with an SMLE poking though the mantlet as the proper weapons were not available.

Yes - think that the Hungarians were also the only ones to mount a Bofors gun in an aircraft (some Me 210/410 IIRC). That features in The Foresight War as well!

Thx.
Seems like everytime I visit another page of your site, I suffer from another bout of KIATS: Kid In A Toy Store syndrome!

With response to a lot of the guns you mention on that page,
I find myself wondering if it would've benefitted the British to have adopted some of the WW1 and interwar designs for WW2 use.
Meaning, instead of a Boys AT rifle, would the Universal Carrier, or others, been better served with any number of those low/medium velocity shell-firing guns?

The manual complexities (or would that have been considered deficiencies?) of the 1.59" Crayford might've seemed a worthless exercise putting it into/onto a light AFV in place of the Boys rifle (or some of those 15mm HMGs the British had), but even its HE (improved prior to WW2?) added advantage could've been a welcome option.

I remember a WW1 photo of two soldiers handling a low-tripod-mounted 1pdr gun of some sort, manually loaded for each round.
Such a weapon could've proven quite useful against any number of obstructions (German MG positions, especially in buildings, and numerous light AFVs and unarmored vehicles), and given the infantry carriers a better accuracy with shells than the typical 50mm/2inch mortars.

Even a single 2pdr "pom pom", or any of those WW1 and interwar automatic aircraft cannon, mounted in a tank turret could've been an interesting option
(again, principally on the grounds of close support work for infantry).

I remember seeing photos of the 37mm gun assembly (weapon and ammo chutes) from P-39 aircraft mounted on the foredeck of US PT boats in the Pacific.
It could've been an interesting support weapon in any number of vehicles that routinely/principally worked with infantry, as well.

After the retiring of the Hurricane tank busters with their cannon, those 40mm S guns could've been good for light AFV installation, too
(fire off a couple bursts and emptying the drum, then retreating to cover/concealment to reload it with a full drum).

The illustration on that page of the various WW1 cartridges suggests any of them could've been suitable light AFV armament for support of infantry, even Home Guard weapons if they were considered too obsolete for frontline work in Europe.
Perhaps even mounted in several of the small landing craft for D-Day and other landings, giving the boats closets to shore a rapid means to silence German MG positions.
And the PTO island-hopping campaign would've been an ideal area to use them, too.
As long as the troops in the infantry vehicles realized they weren't intended to go up against first line armor (tanks and heavy assault guns),
they could've been quite beneficial at close range work for the infantry.

Anything of the sort in your Foresight War story?
I'm definitely going to have to order it shortly.


WRT other posters concerning the Birch Gun, or something quite equivalent for WW2:
The chassis of the Birch Gun was too slow for anything but "walking" along at infantry speed as a bonafide infantry support system.
Also, the gun lacked any serious protection from shrapnel and small arms fire.
An exposed crew would've just been too vulnerable, especially against any snipers trying to kill the gun team to silence the weapon.
The gun itself (18pdr) still could've given useful HE suppport, and at a decent rate of sustained fire.

An 18pdr program on an early WW2 chassis looking like the Bishop might've then given the British the common sense ahead of time to put the 25pdr gun into a rotating turret without sacrifing elevation, and also provided full 360 degree protection.
Or at least come up with something more akin to the Sexton sooner: hampering the 25pdr's range by limiting its elevation was a critical flaw I saw in the Bishop.
And even though the Sexton (like the 105-armed Priest) lacked sufficient overhead cover, it still alowed the 25pdr to get closer to its maximum range than the Bishop allowed...and with more ammo as well.

French light tanks did use a version of the WW I 37mm (1pdr gun) light gun. As did the Renault 1917 which the British had to be aware of.

large feed drums or mechanisms don't go very well in tank turrets. The more volume the feed takes up the less room for crew and/or the larger the turret has to be. overhead or top mounted feeds severly restrict the amount of depression a gun can have without a very high turret roof and this resticts the use of hull down or reverse slope postions. Free swinging weapons in open topped armoured vehicles look impressive but maynot give very good long range accuracy.

Birch gun was actually rated for 45kph top speed which is just a bit above walking speed;) quite useful for shifting from one firing position to another or advancing in time to give fire support to an armoured thrust that might have out run the ability of horse drawn artillery to keep up.
This is the big advantage of SP-guns. the Ability to fire a support mission, pack up and move several miles forward (or even travel for several days at the same speed as the tanks) and set up a new firing position to provide support at a point beyond the range of the original postion. AND do it in a fraction of the time needed by either horse drawn or even truck drawn artillery.
The German army used horse traction for large amounts of it's artillery so even the crudest SP-gun was a significant improvement in mobility.

Many war games don't use big enough maps or model the set up and hitch up times of artillery well enough to show this advantage of SP guns. Players are left to wonder what to do with the SP guns on the small maps and so wind up using the SP guns as assault guns. In real life it might take the time of the entire scenerio in the game to hitch up a battery, move it even a few kilometers and set it back up again.

In history, one of the first times in North Africa that Matilida tanks were shot up by 88s the battery of towed 25pdrs that was supposed to support the attack got bogged down in the sand and didn't reach it's firing position on time. To get back on scheduele it skipped the first of it's fire missions and started with the second target on it's list. Unfortunately it's first target was the position the 88's occuipied. Tracked, SP-guns might have gotten into position on time and while not turning defeat into victory might have prevented the wholesale slaughter that occurred.
Artillery doesn't need to to drive along side of the leading troops. It does need to be able to move from firing position to firing postion in time to keep the leading troops within range of the guns. THAT is keeping up with the advance.

That's why I suggested infantry carriers along the lines of the Universal: generally open topped (not turreted), more intended for actually supporting the infantry right alongside them rather than standing alone and doing recce or antitank support (I don't want to suggest another Stuart light tank or an armored car, although American M20s could've made use of one of these autoguns in its open-topped design, too, not to mention how many American White-pattern open-topped halftracks had all manner of MGs and shell-firing guns mounted onto them as adhoc fire support and infantry help).

The Boys AT rifle mounted in many carriers wasn't very impressive at any ranges. So stepping up to some sort of lightweight pintle-mounted automatic shell firing gun wouldn't really have detracted from the woeful Boys employment (it actually could've made the Universal, or any other lightweights, useful raiders intended to outflank fixed gun emplacements and shoot them up from the flanks...).
Think of it as the WW2 equivalent of a modern day Mk19 40mm MGL: low velocity means it sucks at everything but fixed targets over any measurable range, but still the expolsive shell is deemed worth it.
And like I said, over short direct ranges (within 1/4 mile, give or take), these autoguns would've allowed better accuracy than most infantry mortars, and would've been ideal at shooting the upper works structure of many European buildings where snipers and MGs were positioned.

Not a low-velocity autogun, no. The armoured vehicles I invented for the story tick a few boxes mentioned here though. I decided that it wouldn't be feasible to build a Tiger-destroyer from scratch (UK industry wouldn't have been up to it) so I started with a basic vehicle weighing up to 20 tons which could be adapted to a variety of uses. To save space and weight I copied the layout of the Scorpion/Scimitar light tank family, with an inline engine occupying one side of the nose of the vehicle driving the front sprockets, and the driver sitting alongside (no hull gunner). This dramatically shortens the vehicle and leaves the entire back half clear of obstacles; all manner of different armament or accommodation options can be added without changing the basic vehicle. For an engine (always a problem in early British tanks) I proposed taking one bank of the R-R Merlin - effectively half of a Meteor engine, developing 300 bhp.

The basic tank would deliberately be made underweight initially, and armed with a 40mm gun using Bofors ammo. However, the turret would be designed from the start for upgunning (up to 57mm high-velocity/76mm medium velocity + APDS) while still having room for three in the turret, while the vehicle would be designed to take heavier armour. Other turret options would include a 25 pdr + extra armour for direct support/assault purposes, and an SPAAG with Oerlikon guns in a power-operated turret (also useful for direct fire support). Replacing the turret with a fixed superstructure adds more options: a high-velocity 76mm anti-tank in a limited-traverse mounting behind a thick, well-sloped armoured shield, and a much higher-built SPG allowing full elevation to medium-calibre artillery (4.7" in the story). And of course there would be unarmed command car and APC versions. The intention would be to make the vehicle family useful enough to keep in production throughout the conflict, and (where possible) to recycle the chassis when the tanks became obsolete.

And there would be heavy 6x6 armoured cars taking the same turrets as the tanks, so these could be recycled too.

While this was going on, work would of course be starting on a 40 ton tank with a 600 hp Meteor and a 76mm high-velocity gun initially, with a 102mm medium velocity + APDS to follow...

Except the USA and France both had such a gun in service or available, that is a 37mm low velocity gun - the 'trench gun' and it was found to not have a place in modern warfare. Even the Germans, who effectively had the choice beween the 37mm PaK 36 and the 7.5 le IG 18 chose to use the PAK gun on the pintle of halftracks for infantry support rather than the HE of the 7.5cm weapon.

The USA tried developing the 37mm manpack guns as a light weight infantry support weapon, whilst some saw troop trials they were not well liked or considered effective.

Of course the strangest weapon in this category would have to be the Italian Model 35 Brixia mortar, capable of 25rpm out to 585yards, it was lightweight, firing a 1lb shell but not particularly successful.

I fail to see that an auto version of such a weapon would be any more successful, or capable of an effective higher rate of fire.

A more successful device may have been a vehicle mounted bazooka, firing from a closed breech for higher velocity, with a heat round it could have been effective against AFVs and have a fair secoundary HE capability. (consider the 8.8cm Racketenwerfer 43). Even a PIAT firing from a similar arrangement could have been useful, with more propellant being useable in a vehicle mounted system.

Maybe just bad application? As regimental guns, they would be seriously underpowered. But as Bn or even Co. support they might be quite effective. Might they be made light enough to be employed as HMG?



Not entirely true - 7.5cm HE weapon was also mounted on halftracks and formed part of Battalion firepower. The 3.7cm on Halftracks had more to do with availability, there were more PaK's than iG's and iG's were more urgently needed for infantry divisions of expanding Wehrmacht.

The Brixia 45mm thingie was too weak for a light mortar. But what if it was used in place of today's 40mm AGL, in conjunction with 60mm mortar and MMG's? The same for vehicle applications.

Another weapon for light support applic. might be (in the UK) Polsten, or a single-shot semi-auto weapon using same ammo (kinda RARDEN) for carriers and armored cars.

Cocking and Loading a vehicle mounted PIAT would have been difficult, as the example of the Petard showed. A rocket launcher would have been easier to utilise. Something perhaps along the lines of the "Puppchen"? Extended range, closed breech, firing a rocket?

That is what I said......

quote:



The 8.8cm Racketenwerfer 43 WAS the Puppchen.

Cocking and loading a vehicle mounted PIAT type weapon (ie a spigot mortar) would not necessarily have been difficult, just use a different cocking system and have the 'breech' inside the vehicle.

When thinking about low velocity support weapons there is a rule of thumb on effective ranges. For anti-tank work there was a formula that said effective range was the muzzle velocity plus 10% equeled effective range. What this ment was that for that distance the shell would niether rise above or fall below the line of sight by enough to miss a tank sized target. Of course tanks do vary in size and different shells do loose velocity faster than others. and for support work, windows (or firing slits) atr a lot smaller than tanks standing in the open.
A low velocity gun may be quite accurate in that it shows very little dispersion or might be able to place all ( or a great majority) of it's shots through a good sized window at 1000yds. BUT if it's low velocity trajectory is such that mis-judging the distance by 100yds means you hit either the floor above or below the intended window the guns practical accuracy is somewhat less. Cross winds might affect things also, especially for rockets and other fin stablized projectiles.

Using armoured vehicles that have 8-15mm armour for close support is just not good planning. Yes, it was done, but only because nothing else was available. while such a vehicle is a lot more bullet proof than the human body it is vulnerable to any sort of of heavy support gun the defenders may have been able to scrounge up. In France in 1944 we could be talking about left over French 25mm guns or left over 37mm trench guns like were mentioned earlier. The second line or training units that were issued the German 37mm anti-tank gun while waiting for something better would have had a field day against universal carriers. Providing "flaming datums" isn't really a good use of vehicles.

Daniel:
"not to mention how many American White-pattern open-topped halftracks had all manner of MGs and shell-firing guns mounted onto them as adhoc fire support and infantry help)."

While early enthusiasm for the White half tracks show many prototypes the actual combat usage might show far less. Multipul MGs is one thing, it might not tempt the crew ( or officers) to take on pill boxes or field fortifications with dug in guns. The Halftracks with 75mm pack howtizers, 75mm field guns and 105 howitzers were not intended for close support work nor did they operate that way most of the time. THe howitzers were SP guns pure and simple. they stayed back in normal artillery type positions and provided normal artillery type fire and support. The 75mm Field gun version was intended as a tank destroyer (before the production of full tracked TDs really got under way) and while it worked well enough in ambush it's ability to "slug it out" with any other armoured vehicle was minimal. British did use 2 per armoured car Squadron but even in Italy they were kept to the rear and operated as "private artillery" providing HE or smoke cover for the Armoured cars from a firing position out of line of sight of the expected contact point.

British took a good look at the version equiped with 57mm AT guns and either shipped them on to Russia or pulled the guns and used them for troop carriers. And these were the guys who mounted AT guns on unarmoured trucks! Maybe they had learned something?


Doug:
"Even the Germans, who effectively had the choice beween the 37mm PaK 36 and the 7.5 le IG 18 chose to use the PAK gun on the pintle of halftracks for infantry support rather than the HE of the 7.5cm weapon."

I believe there were 2/3 reasons for this but I could be wrong.
1. the Germans were recycling those 37mm guns, as the 50mm AT gun was issued to the anti-tank platoons/companies the 37mm guns were returned to depot and were available. No such replacement took place for the 7.5 le IG 18. later on some halftraks recieved 75mm/24 guns removed from up-gunned Stug IIIs
2. The halftracks involved with the 37mm were often called platoon leader vehicles. Perhaps the 37mm could be fitted without taking up too much room in the vehicle and dispacing personel?
3. The German 37mm HE round had a very large tracer component. Maybe (ok, I am guessing here) the large tracer was useful for target marking or designating?


ALL:

Given enough time to experiment and tool up my favorite light gun for infantry support would be a version of the German 8cm PAW 600. Maybe in 100mm form? mounted in a Stug style chassis ( or larger vesion of a Hetzer) A few 81mm mortar bombs delivered through the windows would certainly make the occupants of a building rather uncomfortable (100mm version could make it's own windows;) Can fire smoke without much trouble. And tank on the odd tank with at least some chance of success. Enough armour on the front to be safe from all but a serious AT-gun.

I was under the impression that the mechanism of the PIAT (spring-powered spigot) effectively recocked itself upon firing...?

(see 2nd & 3rd paragraphs under the History section in this Wikipedia entry ...)

I remember reading something similar on one of those WW2 "Cards of Knowledge" (c 1977?) that I have stashed away in the attic somewhere, and in some WW2 encyclopedia (or perhaps even one of those TimeLife WW2 books?).

The Wiki article suggests it took considerable manual effort (routinely, 2 men) to initially cock the weapon.
I actually remember a pic from a 1970s era AFV book showing some sort of light tracked AFV with a row of PIATs (6-8?) attached across the front of the vehicle (or maybe the rear, so it could shoot and scoot away faster...), with the intention (just guessing) it might have been a close range ambush/barrage volley.

Again, I wasn't suggesting using the idea as lightweight adhoc anti tank systems.
And I agree with you about the shortcomings of thinly-armored vehicles (small arms fire only and shell fragments).

But just about every armored car design from WW1 onwards is just that: a thin-skinned support vehicle.
And it hasn't really stopped anyone from using them.

Yes, I was thinking along a precursory line to the Mk19 (or perhaps more accurately, closer to the AGS 17 ).
Also note that even if such shells seem rather miniscule in performance, they still can cause more overall damage round for round than solid-bullet MGs.
Also almost equivalently, both the defunct M307 smart grenade MG and even the 30mm M230 series Chainm Guns are, realistically, the culmination of this original concept: short-cased shell-firing guns with more manageable recoil than a lot of the higher-powered cartridges and cases.

I'm looking at it from the perspective of what was used up through WW2, hell even today again with the current trend in 30-40mm MGLs.
Universals were sometimes referred to as Bren Gun carriers, when that was their principal mounted weapon (in addition to infantry dismounts and the 50mm/2inmch mortars).
And how many armored cars used by various nations didn't have anything greater than a "measily" 20mm cannon (and most often, a rather slow-firing one at that).

Such a weapon (lightweight autogun) needn't necessarily be a permanent fixture: it could, like most MGs, be readily dismounted from the vehicle and set up in a good hidey hole area away from the vehicle.
They could be stowed inside the vehicle inconspicuously, so as not to attract extra unwanted attention right up to the moment of firing.
Its employ would've been used little differently than most infantry support MGs.
I'll give that the fuze tech of the day ruled out any sort of proximity or timer fuze for shells that small in that day,
but the extra added explosive punch did have merits: how many Allied aircraft used 20mm guns, that weren't solely firing AP rounds?
If 20mm worked from aircraft at ground targets (not just tanks), why wouldn't these short-cased low-powered shells work from AFVs.

Eployed as how MGLs are today, British LRDGs in Africa could've created some interesing harassment tactics with such weapons,
that couldn't have been accomplished with weapons like the Boys.
Same for American Greyhound-types (M8, M20), British Humbers (some seen with 15mm RollsRoyce HMGs), etc.
Even those halftracks could've providing interesting support opportunities covering infantry advances in tight quarters.
The 57mm & greater guns/howitzers generated much greater recoil.
Those short 37mm guns would've only seemed inadequate until they could've been built with an automatic firing/loading system, again not unlike that P-39 installation.

Looking at several more pages on Tony's site at various WW1 and interwar light guns and ammunition types,
other than being ineffective against WW2 aircraft (unless parked on the ground), what made these guns so inferior for close infantry work,
that Mk19 types today excel at?
(Mk19 ammo has an even lower velocity than a number of those early designs. And as for maximum and sustained rates of fire: Mk19s are almost never fired at max rate (~375rpm?) for several seconds on end, it's always a burst of several rounds, pause & adjust for fall, then fire again.)

A handful of those one-to-three-pound shells (from the auto guns) into a single target could cause a world of hurt more than many WW2 MGs.
Any low recoil auto/semi-auto shell gun that could fire rounds similarly equal in lethality to many hand-throw grenades, and shoot them at least as far as many rifle grenades, seems like it could've fulfilled a good support role.
Mounted in/on light AFVs just would've meant that the infantry wouldn't have been bitching about humping it and its ammo on their backs, other than quickly deploying it off the vehicle to advantagous spots the vehicle couldn't get into (just like how Mk19s are used today...but we really don't necessarily need autoguns that could fire at 4-5 rounds/sec (ever see just how accurately a Mk19 can keep all its rounds on a distant target? That's why we really wouldn't have needed a high rate of fire: the ~100rpm of the "pom pom" would've been plenty).
Even semiautoloading (lessens recoil and vibration further), fed from drums, clips, or whatever, would've been useful.

The 40x79R wouldn't have been a bad starting point.
Two basic types, a low rate of fire auotgun principally intended for vehicle mounting (pintle, or heavy tripod dismount),
and a semiauto/self-loading version along the lines of the Crayford (but could still attack to a pintle mount),
complementing each other along the lines of the 25mm M307 and Barrett Payload Rifle.

Or the 40x158 could've worked, even though its longer length could be recoil-brutal on the shooter of a rifle version.
We may then even have seen something in between (40x100-120?).
Again, I'm looking at this from a perspective of a Mk-19/AGS-17 infantry support gun, with single shot capability.
It could've come down to 4 distinct variants:
an M79 style manually-loaded single shot type "rifle",
a self-loading type, most likely clip fed by a box mag of not more than half a dozen rounds,
a lightweight lower rate of fire autogun version that could be rapidly dismounted from the vehicle (~120rpm max?),
and a higher rate of fire variant distinctly as an internal turret armament not intended to be used dismounted (up to 300 rpm?).

I remember reading a discussion somewhere about the effects of a Skink's 4 20mm guns on a German-fortified building.
A twin mount of these 40mm high ROF light autoguns could've been quite a sight at such a structure, as well.

The PIAT was not the only spigot mortar to be designed. The Blacker Bombard was another, that certainly was not cocked by two men in the way that you describe.

And the naval squid was definitely not cocked in the same was a a PIAT.

The PIAT was not launched by the spigot, the spigot being the equivalent of the 'barrel, in fact the spigit could be considered the projectile and the PIAT shell as the projector....

The firing of the round caused the weapon to re-cock, ready to fire again, as a form of differential recoil.

The PIAT was designed to be cocked by one man, but it wasn't easy.

As the only vehicle mounted spigot mortar demonstrated, loading is quite difficult when you have to fix the centre barrel of the round over the projecting spigot rod. Its not so much the cocking system which is the problem as having to muzzle load the weapon.

Which was because of the size of the projectile.

A small projectle, fired from a more conventially mounted device, would have been easier.

And by the way, there have been two (at least) vehicle mounted spigot mortar systems. I think that you have been referring to the Churchill AVRE, but I would have thought that you would have at least a passing knowledge of that Australian weapons system the Matilda Hedgehog?

quuoting from wiki

Matilda Hedgehog
A naval Hedgehog 7-barrel spigot mortar was mounted in an armoured box on the rear hull of several Australian Matilda tanks. The mortars were hydraulically elevated and electrically fired either individually or in a salvo of six, the fifth tube could not be fired until the turret was traversed to move the radio antenna out of the bomb's flightpath. Each bomb weighed 30 kg and contained 14 kg of high explosive, the range of the bombs was up to 400 metres and aiming accomplished by pointing the entire tank as the mortars had no traverse independent of the hull of the tank.

There is still one of these extant at Pucka. War ended before went into service but the effect on a Japanese bunker would have been tremendous.