Tank & AFV armament alternatives, 1935-45

[tomo pauk]

As a companion to the alternative light and AT guns
Be these all-new guns that never were installed on the tanks, or nip & tuck job on the historical guns.. Also includes the plausible ammo choices, but also the changes in turret size to fit the guns more conveniently, as much as the turret ring allows. Since the AFVs are also here, they might have an easier time to get a big whacking gun installed.
Both AP and HE abilities are important; in what ratio, it will depend on the country doctrine and perceived threats.

To start with the French: The 75mm howitzer from the Char B as a turret gun, with a bit longer barrel (it was 17 cals; even at 22-24 cals it will not be too long for the ammo it historically fired); it might get the plain AP shot at 500 m/s. It will need a 2-men turret for the Somua 35 tank - this will be seen as the biggest drawback by the bean counters in the Government.

 

[EwenS]

Another point to consider. What degree of depression does your gun have to have and across how great an angle? Becomes important when you crest a ridge.

War Office insistence on 10 degree depression over the whole 360 degrees drove the design of the A30 Challenger turret. Forced a higher trunnion height and space above the breech end due to the long engine deck, large breech and recoil of the 17pdr. Also designed to accommodate two loaders deemed necessary to manoeuvre the heavy 17pdr rounds in the confines of the turret.

All suddenly deemed unnecessary by late 1943 when the Firefly began development. Can't find details for following Comet & Centurion. Discussion here of pros and cons on tank height.


View: https://www.youtube.com/watch?v=zK6suAebD-M

 

[tomo pauk]

War Office insistence on 10 degree depression over the whole 360 degrees drove the design of the A30 Challenger turret. Forced a higher trunnion height and space above the breech end due to the long engine deck, large breech and recoil of the 17pdr. Also designed to accommodate two loaders deemed necessary to manoeuvre the heavy 17pdr rounds in the confines of the turret.

All suddenly deemed unnecessary by late 1943 when the Firefly began development. Can't find details for following Comet & Centurion. Discussion here of pros and cons on tank height.

I've suggested in the earlier thread that British would've been perhaps better off with the 75/77mm HV gun developed in lieu of the 17 pdr, since the less bulky gun would've been easier and faster to install on the existing tanks. On the Sherman certainly, and the A30 can be made more of a tank than a turreted TD. Perhaps the installation on the Churchill would've been possible. Shorted and lighter ammo will also help within the confines of the tank.

For the earlier British tanks (before 1940), something that fires the ammo from the 13 pdr 9cwt or, even better, the 12pdr 12cwt would've been a world beater. The late 1930s British tanks had the sizable turrets that would've made the installation of the ( for the day) potent guns feasible.

 

[EwenS]

I've suggested in the earlier thread that British would've been perhaps better off with the 75/77mm HV gun developed in lieu of the 17 pdr, since the less bulky gun would've been easier and faster to install on the existing tanks. On the Sherman certainly, and the A30 can be made more of a tank than a turreted TD. Perhaps the installation on the Churchill would've been possible. Shorted and lighter ammo will also help within the confines of the tank.

For the earlier British tanks (before 1940), something that fires the ammo from the 13 pdr 9cwt or, even better, the 12pdr 12cwt would've been a world beater. The late 1930s British tanks had the sizable turrets that would've made the installation of the ( for the day) potent guns feasible.

You mean an earlier A43 Black Prince?

Black Prince (tank) - Wikipedia

en.wikipedia.org

Another factor to bear in mind is rail transport, which limited the overall width of British tanks (Churchills had to have their air filters removed for example). Problem is not just the width of flatcars to carry them, but tunnels (width, curvature of roofs etc) and clearances on station platforms they passed through.

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Width seems to have been less of an issue on the Continent or within the USA.

 

[tomo pauk]

You mean an earlier A43 Black Prince?

My cunning plan is to avoid the Black Prince design job, since that was a whole new 50-ton tank.

Another factor to bear in mind is rail transport, which limited the overall width of British tanks (Churchills had to have their air filters removed for example). Problem is not just the width of flatcars to carry them, but tunnels (width, curvature of roofs etc) and clearances on station platforms they passed through.

The historical width of the British tanks is fine, IMO.

 

[bf109xxl]

Perhaps the installation on the Churchill would've been possible.

The 76mm ZIS-5 was mounted on the Matilda. The Soviets noted the tightness of the turret, but this was a tribute to inter-organizational conflict rather than a real shortcoming.

 

[Shortround6]

Both AP and HE abilities are important; in what ratio, it will depend on the country doctrine and perceived threats.

For the British this means rethinking their entire doctrine/philosophy.
British doctrine was developed between the wars for shooting while moving. Hey! it worked for ships
British used the gunner as a human vertical stabilizer.

There is no hand wheel for elevation. The gunner used his body to control the elevation of the gun as the tank bounced over the ground. This did not work well even with a small/light 2pdr gun (287lbs) or it's cousins (the 3.7in mortar/howitzer and the 3in howitzer). But it was this easy (?) control the British were looking for. They did keep the shoulder control on some (all?) of the 6pdr armed tanks. (700-800lb gun). What they did on the 75mm guns I don't know. I do hope they used an elevating wheel on the 95mm howitzer tanks although according to one account the accuracy was horrible. How much may have been due to the elevation system I don't know.
The gun in the Comet was 1500lbs and trying to control that weight, even if extremely well balanced, with a crewman's shoulder was going to be difficult if not impossible.
Of course by the time the Comet showed up the tanks were stopping to fire and had gone to at least a hand wheel. I am trying to figure out if they had powered elevation.

It will need a 2-men turret for the Somua 35 tank - this will be seen as the biggest drawback by the bean counters in the Government.

The French tanks needed two man turrets for their 37mm and 47mm guns. A tank with a one man or most two man turrets was a self propelled gun.
Even a German Stug had a commander who was thinking about where the enemy was and how to maneuver his vehicle to best advantage. A commander that was playing gunner was doing neither thing until he had dealt with the target in his gun sight and a commander that was acting as a loader might not even be doing that.
Sticking bigger guns in in French tanks does not solve the command/control problem.
Please note that the two man turrets (with poor vision) in the early soviet T-34s were partially responsible for the rather dismal performance in combat.
A tank with the specs of the T-34 (speed, fire power, armor) should have done much better against the Germans than it did. Poor training is one thing. Poor training and trying to do too many jobs at once and having poor vision at the same time is a real recipe for poor performance.

 

[bf109xxl]

Please note that the two man turrets (with poor vision) in the early soviet T-34s were partially responsible for the rather dismal performance in combat.
A tank with the specs of the T-34 (speed, fire power, armor) should have done much better against the Germans than it did. Poor training is one thing.

Poor training was the major problem, more significant than all the others listed above. The next problem was the lack of radio communications on the most of the early T-34s. But the other disadvantages mentioned ( tightness and poor vision) also greatly reduced the combat efficiency.

Poor training and trying to do too many jobs at once and having poor vision at the same time is a real recipe for poor performance.

The situation with the training of Soviet tankers was so deplorable that sometimes it is hard to believe the facts. Some analysis of the training problems in the Soviet tank forces before the war was given in the book "The T-34 goes to war" by Ulanov and Shein. A more comprehensive analysis can be found in the book "Order in the tank forces? Where Stalin's tanks disappeared to" by the same authors', unfortunately only in Russian.

 

[z42]

Both AP and HE abilities are important; in what ratio, it will depend on the country doctrine and perceived threats.

As S Shortround6 wrote, doctrinal deficiencies really are the key here. Without a doctrinal need, there is little to drive the development of appropriate systems. In retrospect it's somewhat perplexing, some combined-arms exercises in the 1930'ies ought to have showed that a tank needed the ability to shoot either AP or a useful HE load depending on what is encountered. The British in particular, but seems many others also committed the same mistake. For the early-mid war, I think the sweet spot for a tank gun would be something like a medium velocity 75mm gun (L40 barrel length or thereabouts, for a MV in the 600-700 m/s range). Big enough to pack a decent HE load, and sufficient AP capability to penetrate the early-mid war tanks. Later in the war, as tanks get better armor, move up to a high velocity 75mm gun, or even something in the 85-90mm range. And yes, put that gun in a three man turret so that the commander can concentrate on doing commander-y things instead of crewing the gun.

 

[yulzari]

For the British this means rethinking their entire doctrine/philosophy.
British doctrine was developed between the wars for shooting while moving. Hey! it worked for ships
British used the gunner as a human vertical stabilizer.
There is no hand wheel for elevation. The gunner used his body to control the elevation of the gun as the tank bounced over the ground. This did not work well even with a small/light 2pdr gun (287lbs) or it's cousins (the 3.7in mortar/howitzer and the 3in howitzer). But it was this easy (?) control the British were looking for. They did keep the shoulder control on some (all?) of the 6pdr armed tanks. (700-800lb gun). What they did on the 75mm guns I don't know. I do hope they used an elevating wheel on the 95mm howitzer tanks although according to one account the accuracy was horrible. How much may have been due to the elevation system I don't know.
The gun in the Comet was 1500lbs and trying to control that weight, even if extremely well balanced, with a crewman's shoulder was going to be difficult if not impossible.
Of course by the time the Comet showed up the tanks were stopping to fire and had gone to at least a hand wheel. I am trying to figure out if they had powered elevation.

The shoulder man powered elevation stemmed from the use of naval guns in the Great War British tanks and those needed it as they were for small vessels which pitched and rolled on the sea and were for close anti torpedo boat firing so it suited the slowly moving early tanks and continued to be practical in the lightweight inter war guns. It worked well in trials and with experienced gunners in action. However it meant that the gun had to be balanced from a point well back to keep the load on the gunner's shoulder minimised and thus we got the British habit of internal gun mantlets. Hand wheel or powered elevation could cope with more weight so they could put the mantlet outside the turret which meant that they did not have a gaping hole in the front of the turret and the gun was, as a whole, mounted further forward so it took up less room and/or had more space for recoil control. A minor other effect was that the internal mantlet breech needed more height than with an external mantlet to get good depression as it was further back in the tank.

The shoulder system was an effective choice at the time and allowed a good gunner to fire on the move on rolling ground but less so as guns grew larger and heavier.

 

[tomo pauk]

The French tanks needed two man turrets for their 37mm and 47mm guns. A tank with a one man or most two man turrets was a self propelled gun.
Even a German Stug had a commander who was thinking about where the enemy was and how to maneuver his vehicle to best advantage. A commander that was playing gunner was doing neither thing until he had dealt with the target in his gun sight and a commander that was acting as a loader might not even be doing that.
Sticking bigger guns in in French tanks does not solve the command/control problem.

Agreed.

Please note that the two man turrets (with poor vision) in the early soviet T-34s were partially responsible for the rather dismal performance in combat.
A tank with the specs of the T-34 (speed, fire power, armor) should have done much better against the Germans than it did. Poor training is one thing. Poor training and trying to do too many jobs at once and having poor vision at the same time is a real recipe for poor performance.

There was a lot of other factors in play with the T-34 not giving the edge to the Soviets that, looking by it's 'book' values, it should have.
German crews were not just better trained, a good deal of them have had some sort of war experience. German tanks provided the crew, and especially the commander, with numerous ports and periscopes to look around and take stock of the situation. Radios were probably better and more widely spread in the Heer of 1941. Combined arms approach was German bread and butter, it will take until 1943 for the Soviets to came in close in that regard?
Air superiority (or lack of it for the Soviets) was a factor in 1941-42. Germans were also much faster to adapt to the new situation on the ground, while any kind of independent, non-scripted action was a surefire way to end up badly for a person that ordered it in the Red Army of 1941-42.

Even the KV-1, with it's 3-men turret and even better armor was not a good dam to forestall the German advances, the isolated successes notwithstanding.

As S Shortround6 wrote, doctrinal deficiencies really are the key here. Without a doctrinal need, there is little to drive the development of appropriate systems. In retrospect it's somewhat perplexing, some combined-arms exercises in the 1930'ies ought to have showed that a tank needed the ability to shoot either AP or a useful HE load depending on what is encountered. The British in particular, but seems many others also committed the same mistake.

French tanks were all outfitted both with HE and AP shells, same for the Polish, Czech and German and Soviet tanks. The later two even had the HE throwers in the Pz-IV and different versions of the BT tanks, respectively, while the French went the extra mile with the two-gunned Char B, where both guns had the AP and HE ammo.

For the early-mid war, I think the sweet spot for a tank gun would be something like a medium velocity 75mm gun (L40 barrel length or thereabouts, for a MV in the 600-700 m/s range). Big enough to pack a decent HE load, and sufficient AP capability to penetrate the early-mid war tanks.

Ironically, all the countries that we think of being capable for tank production have had the gun of these properties. Even the ww1 leftovers would've been great as the tank guns.
For the French, assuming they survive the 1940 onslaught, a tank gun based on the newer AA pieces (~700 m/s) could've been useful into the 1960s with new ammo. Similar with the Soviets, where the F22 gun was rated for 1400g or propellant vs. the F34 for just 1080g. Germans also have several 75mm guns that can be very useful for the ww2 with 600 m/s and more; they can adopt the Czech guns for this role, too. Italians can compete, too - and they tried, if too late and too few to matter; same with the Japanese.

Something for the Germans - once the Czech 47mm gun is available, focus on it as the weapon for the Pz-III, forget the short 5cm tank gun. The long French 47mm ATG would've also been fine for the Pz-III.

 

[Shortround6]

The shoulder system was an effective choice at the time and allowed a good gunner to fire on the move on rolling ground but less so as guns grew larger and heavier.

And what happens when you have average gunners?
Not poor, just rushed in building up desired forces.

And what happens at longer ranges?
Like much over Point Blank when putting the cross hairs just over the enemy target is no longer enough.

What happens at the longer ranges when the average gunner needs repeat shots and can't just dial the elevation up or down, even with the tank sitting still?

British used a lower magnification sight, perhaps to allow for a wider field of view for shooting on the move.
But this meant more difficulties shooting at long range.
By the way, in desert long range for 2pdr armed tanks was anything much over 800 yds. Maybe the expected ranges in France were less but they shot themselves in both feet with this one and kept throwing the crutches away for several years. Apparently I was in error earlier about shoulder control being used with close support howitzers. The Chieftain's video of the A10 CS tank shows hand wheel elevation for the 3.7in Mortar. Never let actual experience get in the way of good doctrine, or even mediocre doctrine........................heck, even down right crappy doctrine.

 

[z42]

French tanks were all outfitted both with HE and AP shells, same for the Polish, Czech and German and Soviet tanks. The later two even had the HE throwers in the Pz-IV and different versions of the BT tanks, respectively, while the French went the extra mile with the two-gunned Char B, where both guns had the AP and HE ammo.

My point was that the medium velocity ~75mm gun would have removed the need for separate AP and HE gun equipped tanks. Keeps the momentum going when you don't have to fall back and wait until the right kind of tank comes along to deal with whatever threat you encountered.

Ironically, all the countries that we think of being capable for tank production have had the gun of these properties. Even the ww1 leftovers would've been great as the tank guns.
For the French, assuming they survive the 1940 onslaught, a tank gun based on the newer AA pieces (~700 m/s) could've been useful into the 1960s with new ammo. Similar with the Soviets, where the F22 gun was rated for 1400g or propellant vs. the F34 for just 1080g. Germans also have several 75mm guns that can be very useful for the ww2 with 600 m/s and more; they can adopt the Czech guns for this role, too. Italians can compete, too - and they tried, if too late and too few to matter; same with the Japanese.

Indeed many such guns were available, or were developed in that time frame. Nothing technical prevented something like the US M3 or British QF 75mm etc. from being developed earlier either, they just didn't figure out that something like that was wanted earlier.

Something for the Germans - once the Czech 47mm gun is available, focus on it as the weapon for the Pz-III, forget the short 5cm tank gun. The long French 47mm ATG would've also been fine for the Pz-III.

Equipped with the retrospectoscope, the Germans could have developed a medium velocity 7.5 cm cannon, something between the 7.5cm KwK 37 (L24) and KwK 40 (L43/48), equip Pz IV with that. That would have given them a decent general purpose tank capable of dealing with most threats until the Panther shows up. Stop producing Pz III's, keep the Pz III chassis production line going for Stug's, SP artillery, maybe a SP AA, and support vehicles.

 

[Shortround6]

There was a lot of other factors in play with the T-34 not giving the edge to the Soviets that, looking by it's 'book' values, it should have.
German crews were not just better trained, a good deal of them have had some sort of war experience. German tanks provided the crew, and especially the commander, with numerous ports and periscopes to look around and take stock of the situation. Radios were probably better and more widely spread in the Heer of 1941. Combined arms approach was German bread and butter, it will take until 1943 for the Soviets to came in close in that regard?
Air superiority (or lack of it for the Soviets) was a factor in 1941-42. Germans were also much faster to adapt to the new situation on the ground, while any kind of independent, non-scripted action was a surefire way to end up badly for a person that ordered it in the Red Army of 1941-42.

Even the KV-1, with it's 3-men turret and even better armor was not a good dam to forestall the German advances, the isolated successes notwithstanding.

Agreed, just pointing out that sticking a more powerful gun in a tack doesn't solve many of the other problems. It might help patch over a few but doesn't solve the fundamental problems. And may introduce 1 or 2 of it's own.

French tanks were all outfitted both with HE and AP shells, same for the Polish, Czech and German and Soviet tanks. The later two even had the HE throwers in the Pz-IV and different versions of the BT tanks, respectively, while the French went the extra mile with the two-gunned Char B, where both guns had the AP and HE ammo.

Now we may have to identify what is an HE thrower
A grenade thrower (37mm) or a fortification/building destroyer.
EVERYBODY in the world had grenade throwers (37mm HE) in their tanks except for the British.
Fortification/building destroyer weapons not so much, you are correct.

Ironically, all the countries that we think of being capable for tank production have had the gun of these properties. Even the ww1 leftovers would've been great as the tank guns.
For the French, assuming they survive the 1940 onslaught, a tank gun based on the newer AA pieces (~700 m/s) could've been useful into the 1960s with new ammo. Similar with the Soviets, where the F22 gun was rated for 1400g or propellant vs. the F34 for just 1080g. Germans also have several 75mm guns that can be very useful for the ww2 with 600 m/s and more; they can adopt the Czech guns for this role, too. Italians can compete, too - and they tried, if too late and too few to matter; same with the Japanese.

Something for the Germans - once the Czech 47mm gun is available, focus on it as the weapon for the Pz-III, forget the short 5cm tank gun. The long French 47mm ATG would've also been fine for the Pz-III.

The WW I leftovers are not that great, they may form a basis but the sooner that basis is left behind the better off the tank forces are.
The majority of the Worlds WW I field guns were around 500m/s velocity (give or take) with the French 75 being out of that range on the high side.
Which often means that you are looking at naval/coast artillery guns as a starting point for high velocity. And the Navy and Coastal artillery men were not worried about weight, at least not much.
Most of what can be saved is just the bore dimensions and the tooling for both barrel making and shell production.
I would be very careful of the whole 75-77mm size of of guns. The British went from 13pdr (actually 12.5) to 17pdr projectiles.
The British 13pdr 9cwt gun is an 18pdr (83.4mm) with the barrel fitted with a liner to bring the bore down to 76mm and keep the 18pdr cartridge case. Also used the 13pdr (12.5lb) shell so velocity should be taken with a sizable dose of salt when figuring out AT projectile velocity. The 3in 20cwt gun at 2500fps was also using the 12.5lb HE shell and when using a later 16lb shell, velocity dropped to 2000fps. This thing shows it's ex naval heritage (anti-torpedo boat/destroyer gun) with it's 2250lb barrel and breech weight, well over double the weight of the 75mm gun in the Sherman. A lot of books seem to gloss over the "adapted from" part


The next problem for 1930s tank is just trying to stuff a 75mm field gun into the existing tanks of the time. It is not a question of making the gun fit with a tape measure. It is having the tank (and the crew) survive firing the gun and doing so repeatedly. Only the French had tanks over 20 tons (the Somua just misses depending on data) and the German MK IV didn't hit 20 tons until the D model after about 210 earlier tanks have been built. MK III didn't exceed 20 tons until the G model.
Not saying that 20 tons is a magic number (US M24 tank went 20.3 short tons) but in a landscape of 15-18 ton tanks (and smaller) trying to use 75mm high velocity guns is a problem. The recoil breaks things (including crew) and sometimes firing at 90 degrees on a hill (sideslope) can get more exciting than people bargained for. There is a reason that the Germans used muzzle brakes. You also have to juggle the fire power of a few shots with the firepower over an extended period of time. The M24 held 48 rounds of distinctly medium sized ammo. Yes in modern thinking we can ditch the bow gunner and fill his seat with ammo
The A 10 CS tank dropped from about 100 rounds of 2pdr to about 44 rounds of 3.7in Mortar ammo and that ammo had a really short cartridge case.

Getting the desired number of bigger, heavier (but better) tanks out of the treasury depts of most countries was going to be very difficult and now you have to design and buy no transport equipment and new bridging equipment.
Everybody wound up with bigger, heavier, more capable tanks but the road was bumpy and expensive.

 

[bf109xxl]

Air superiority (or lack of it for the Soviets) was a factor in 1941-42.

In 1941-1942 the major losses of Soviet tanks were from artillery fire, while losses from enemy aircraft were rather insignificant. As well as, probably, throughout the war in general, although some episodes with high losses from the Luftwaffe actions occurred, but later - when the "Stukas" G appeared at the front.

 

[bf109xxl]

German crews were not just better trained, a good deal of them have had some sort of war experience. German tanks provided the crew, and especially the commander, with numerous ports and periscopes to look around and take stock of the situation.

Situation: the tankers have to break through the village defenses, at night the engineers have to demine the approaches to the enemy positions, and the infantry has to follow the tanks, preventing the enemy infantry from using anti-tank grenades/molotov cocktails/etc.
But by morning, the engineers fail to demine, which they don't report to the tanks due to lack of radio communication, and the infantry just lie down in the snow and don't follow the tanks on the offensive.
As a result, tanks suffer heavy losses on mines, and those tanks that managed to break through enemy positions are destroyed by grenades, etc.
This is not an exceptional but rather a typical situation in which no optical devices or other technical means will help. A very large percentage of Soviet tanks were lost in such situations. Therefore, it is necessary to evaluate the Soviet equipment by its losses with great caution. I think that in the same conditions with the same equipment the Germans would have achieved completely different results only because of better interaction between the different troops on the battlefield.

 

[tomo pauk]

I would be very careful of the whole 75-77mm size of of guns. The British went from 13pdr (actually 12.5) to 17pdr projectiles.
The British 13pdr 9cwt gun is an 18pdr (83.4mm) with the barrel fitted with a liner to bring the bore down to 76mm and keep the 18pdr cartridge case. Also used the 13pdr (12.5lb) shell so velocity should be taken with a sizable dose of salt when figuring out AT projectile velocity. The 3in 20cwt gun at 2500fps was also using the 12.5lb HE shell and when using a later 16lb shell, velocity dropped to 2000fps. This thing shows it's ex naval heritage (anti-torpedo boat/destroyer gun) with it's 2250lb barrel and breech weight, well over double the weight of the 75mm gun in the Sherman. A lot of books seem to gloss over the "adapted from" part

The heavy '3in class' projectiles don't appear on the map of wide use before 1942. So our valiant guns will be probably having the projectiles of 6-6.5 kg. I don't see that as a problem.
There were guns that are easier to fit than the other ones. I've already suggested the two guns that will be easier to fit for the British, than the big & heavy 20cwt:

13 pdr 9cwt or, even better, the 12pdr 12cwt

British can move on the 76 mm HV as the next step by 1939 (start of development) to 1942 (initial use).

The next problem for 1930s tank is just trying to stuff a 75mm field gun into the existing tanks of the time. It is not a question of making the gun fit with a tape measure. It is having the tank (and the crew) survive firing the gun and doing so repeatedly. Only the French had tanks over 20 tons (the Somua just misses depending on data) and the German MK IV didn't hit 20 tons until the D model after about 210 earlier tanks have been built. MK III didn't exceed 20 tons until the G model.

Size of the tank was a greater bane for the more powerful guns than the weight of the tank being low. Big (for the time) guns will drive the size of the turret up, and there was just a minority of the tanks able to 'swallow' the upgrade without much of the problems. How much of the support from the higher ups for the major up-gunning jobs in some countries - we can only guess.
Eg. see the small and light Valentine - at the end, it carried the gun similar to what the tanks of double the weight were normally carrying, but it meant that a lot had to be sacrificed, like the crew number and comfort, as well as the number of rounds for the MG (luckily the external size of the 75mm ammo was the same as of the 6pdr, so the cannon ammo count remained the same as with the tanks armed with the 6pdr, but just the half of the 2pdr), while being without a belt-fed pintle-mounted MG (let alone the HMG).
Luckily, with Valentine the designers at least have the common sense to design the mantlet to go beyond the turret ring for the 75mm, so the gun was able to fit. Cromwell had no such luck when the 77mm HV was mooted for it, and same goes for the Comet with the 17pdr.

Not saying that 20 tons is a magic number (US M24 tank went 20.3 short tons) but in a landscape of 15-18 ton tanks (and smaller) trying to use 75mm high velocity guns is a problem. The recoil breaks things (including crew) and sometimes firing at 90 degrees on a hill (sideslope) can get more exciting than people bargained for. There is a reason that the Germans used muzzle brakes. You also have to juggle the fire power of a few shots with the firepower over an extended period of time. The M24 held 48 rounds of distinctly medium sized ammo. Yes in modern thinking we can ditch the bow gunner and fill his seat with ammo
The A 10 CS tank dropped from about 100 rounds of 2pdr to about 44 rounds of 3.7in Mortar ammo and that ammo had a really short cartridge case.

As noted above, even at under 18 tons, the 75mm of good ballistics was not a problem for the integrity of a tank.
Muzzle brakes were used before 1930s, so there is a thing to add to the gun in the design phase. If the muzzle barrel is later recognized as not needed, remove it. Again, there was no muzzle brake on the 75mm on the Valentine.
There was a lot of under 10 ton vehicles where the much more powerful 3in category guns were installed. The main problem was the armor protection, not the worry that the vehicle will flip when guns were fired. Granted, these were not able to swipe more than 20 deg aside, but still.

Also note that, thus far, nobody suggested the high velocity 75mm guns for the under-20 ton tanks.

Getting the desired number of bigger, heavier (but better) tanks out of the treasury depts of most countries was going to be very difficult and now you have to design and buy no transport equipment and new bridging equipment.
Everybody wound up with bigger, heavier, more capable tanks but the road was bumpy and expensive.

A lot of countries made a too small a gun on a too big/too heavy a tank. Matilda II is probably the most often-tanked example, but, if a 37-40mm gun is a good fit for an 8-10 ton, the 14-15 ton tank will be needed something more potent, let alone a 18-20 ton type.

 

[Shortround6]

The heavy '3in class' projectiles don't appear on the map of wide use before 1942. So our valiant guns will be probably having the projectiles of 6-6.5 kg. I don't see that as a problem.
There were guns that are easier to fit than the other ones. I've already suggested the two guns that will be easier to fit for the British, than the big & heavy 20cwt:

A 6.5 kg projectile is about 14% heavier than the old 12.5lb projectile and will have at least 14%more recoil (add a little bit for extra propellent) at the same velocities.

Eg. see the small and light Valentine - at the end, it carried the gun similar to what the tanks of double the weight were normally carrying, but it meant that a lot had to be sacrificed, like the crew number and comfort, as well as the number of rounds for the MG (luckily the external size of the 75mm ammo was the same as of the 6pdr, so the cannon ammo count remained the same as with the tanks armed with the 6pdr, but just the half of the 2pdr), while being without a belt-fed pintle-mounted MG (let alone the HMG).

OK, lets see,
Valentine at 20 tons with 3 men and holding 53 rounds of 75mm ammo (?) and 1575 rounds of mg ammo. Vision very bad. speed slow. armor good.
M4A1 at 34 tons with 5 men and holding 90 rounds of 75mm ammo, 4750 rounds of mg ammo, Vision not good, speed medium, armor medium/good.

The Valentine was not a good bargain. It was what they could make in a hurry with without extensive retooling of the factory.
In the 1930s and using a clean sheet of paper would it be the way to go?
And what 'doctrine' does it fit?
Cruiser/exploitation or infantry support (we know it was designed for infantry support)

Japanese type 89 medium tank 14 tons, 100 rounds of 57mm ammunition 2745 rounds of mg ammo, vision bad (very?), speed slow, armor poor. Production started in 1931.

20 tons, 200 rounds of 37mm ammo, 12,250 rounds of mg ammo, vision poor (when closed up), speed medium. Armor poor.

There was a lot of under 10 ton vehicles where the much more powerful 3in category guns were installed. The main problem was the armor protection, not the worry that the vehicle will flip when guns were fired. Granted, these were not able to swipe more than 20 deg aside, but still.

It kind of is the point. Tanks want to have 360 degrees of rotation, what they can get away with with limited traverse is not what they can get away with at 90 degrees, anymore than what you can get away with with a towed AT gun using split trails vs a 360 degree carriage.
American built the M3 medium while they designed the rotating turret for the M4. Granted the M3 was a rush job but they already had the chassis/drive line.
Of interest. The Valentine was within an inch of the width of the US M2 medium tank in the photo above. Hight of the gun above the ground when cross firing is also important.

Just for fun

 

[tomo pauk]

OK, lets see,
Valentine at 20 tons with 3 men and holding 53 rounds of 75mm ammo (?) and 1575 rounds of mg ammo. Vision very bad. speed slow. armor good.
M4A1 at 34 tons with 5 men and holding 90 rounds of 75mm ammo, 4750 rounds of mg ammo, Vision not good, speed medium, armor medium/good.

The Valentine was not a good bargain. It was what they could make in a hurry with without extensive retooling of the factory.

Valentine is a bargain. When 1st available, it was half a price of the Matilda II with the same gun, and just a slightly less armor thickness. It was also what the British industry was able to churn out in good numbers, without the dip in reliability (unlike a lot of cruiser tanks before the Comet).

In the 1930s and using a clean sheet of paper would it be the way to go?
And what 'doctrine' does it fit?
Cruiser/exploitation or infantry support (we know it was designed for infantry support)

Alternative Valentine you mean?
It would've been the 2nd tier among the British tanks, the 1st tier populated with 27-30 ton tanks with a decent engine (350-400 HP V12 preferably), with Horstmann suspension, and a 12-13 lb gun. Made in the well-outfitted factories.
The 2nd tier tank can be powered by the twinned bus engine that historical Matilda used, armed with the spin-off from the 6pdr 10cwt gun, also preferably with Horstmann suspension. Aim at 20 tons. Made in the factories that are less well outfitted, and also in Canada. It can support infantry all day long.

 

[MikeMeech]

A 6.5 kg projectile is about 14% heavier than the old 12.5lb projectile and will have at least 14%more recoil (add a little bit for extra propellent) at the same velocities.

OK, lets see,
Valentine at 20 tons with 3 men and holding 53 rounds of 75mm ammo (?) and 1575 rounds of mg ammo. Vision very bad. speed slow. armor good.
M4A1 at 34 tons with 5 men and holding 90 rounds of 75mm ammo, 4750 rounds of mg ammo, Vision not good, speed medium, armor medium/good.

The Valentine was not a good bargain. It was what they could make in a hurry with without extensive retooling of the factory.
In the 1930s and using a clean sheet of paper would it be the way to go?
And what 'doctrine' does it fit?
Cruiser/exploitation or infantry support (we know it was designed for infantry support)
View attachment 808541
Japanese type 89 medium tank 14 tons, 100 rounds of 57mm ammunition 2745 rounds of mg ammo, vision bad (very?), speed slow, armor poor. Production started in 1931.
View attachment 808542
20 tons, 200 rounds of 37mm ammo, 12,250 rounds of mg ammo, vision poor (when closed up), speed medium. Armor poor.

Hi
Surely the B&W is not the Japanese Type 89 but the US M2, the type of tank the US Army would have gone to war with in 1939-40, if that had been decided upon:

(source page 173, 'Tanks of the World 1915-1945' by Chamberlin and Ellis)
The late entry into the war was a bit of a godsend for the US Army.
Mike