For discussion of field guns, anti-tank and anti-aircraft guns.

Work on the 17 pdr to make it the Allied 88!

Even without the advantage of a time traveler, I think the logical approach with regards to anti-armor prewar would be to see where our own capabilities lie with regards to the upper weight limit at which a powertrain (especially transmission) could be made to function reliably.   Add in a margin for advancement, and then you'll have an idea of what weight class of threats we may see in the near future, and thus an idea of what sort of armor we would be facing.  Develop our own anti-armor weapons (and vehicles to carry them) with that in mind, plus some margin. 

My suspicion is that a 10 year study (1935-1945) would conclude that typical armor threats faced could be made reliable up to the 35 ton range by 1945 (up to 45 tons after margins depending if R&D is in peacetime or wartime footing), which we can use to estimate frontal protection bounds (both thickness and slope), and while heavier tanks may be encountered, they are unlikely to be reliable enough to be seen in untenable quantities. 

That threat estimation would serve to shape the design and development of our own armor, and the result would be entering the war with relatively over-armed medium tanks relative to historical, but are future resistant so the odds of having to interrupt production, at least from that aspect, is minimized.
 

I know that, in all aspects of armament, developments in the UK were handicapped by a lack of resources most particularly in skilled and experienced designers. Vickers was the only firm involved in tank building in the 1930s and they generally made relatively small and unimpressive vehicles. They did apply some evaluation to transmissions and suspensions, but were handicapped by the lack of sufficiently powerful and reliable engines. On the other hands, France was making some big and heavy vehicles, indicating what might need to be dealt with.

I think that a planned progression in AT gun power to match any feasible level of armour was probably the most sensible approach, and that is what was done in reality: the main problem was the delay in the introduction of the 6 pdr caused by the huge losses of 2 pdr at Dunkirk.

What I'm thinking of for later tank guns (3 inch and up, calibre) is ammunition which combines very good penetration with a large and effective HE shell (the general idea is discussed here). The case is optimised for sub-calibre AP, therefore with a relatively large calibre and a long, fairly straight case. The AP projectiles (APCR/HVAP initially, APDS to follow) are quite short, but the HE would weigh around 20lbs. It would be very long, fired at a low velocity permitting thin projectile walls for maximum HE capacity. Most of the shell would therefore be buried in the case.

The same principle would follow with the next step up, the 4 inch calibre. This would basically be the 17 pdr AT gun case necked up to 101.6mm. The increased bore diameter would allow an increase in muzzle energy, possibly from 3,000 kJ to around 4,000 kJ for the AP. The long, thin-walled HE shell might weigh in at around 40 lbs, containing maybe 6 lb HE - appreciably more than the 105mm howitzer.

Are you intending to have the above 4" gun as your main field artillery howitzer? Or would you develop the 25 pounder as the original time line?

Or a 105 mm that would be compatible with the U.S. Army's main howitzer in World War II?

British artillery projectiles were noted for low H.E. content compared to U.S. shells. I've read this was due to lower quality steel used in British projectiles.

For instance:  

U.S. 105 mm had 4.8 pounds of H.E. in a 33 pound projectile                         15% fill      vs

German 105 mm howitzer 4 pounds H.E. in 33 pound projectile                    12% fill

British 4.5 " (115mm) gun with 4.5 pounds of H.E in a 55 pound projectile    8% fill

British 25 pounder 2 pounds of H.E. in a 25 pound projectile                          8 % fill

Your 40 pound projectile seems relatively heavy for a 100mm projectile. 6 pounds of H.E. in a projectile would be 15%. About right for a U.S. projectile but high for a British one.

Would you switch to higher quality steel for British artillery projectiles? Could  British industry in the late 1930's support that?

17thfabn said:

British artillery projectiles were noted for low H.E. content compared to U.S. shells. I've read this was due to lower quality steel used in British projectiles.

The brits ran out of shells in the first world war so settled on much lower quality steel and thicker walls would add more fragments to avoid bottlenecks in production. They did change that midwar with the 80 pound 5.5 shell which was 15% burster charge. 

Tony's 40 pound shell would only have about a 3 pound charge due to the need to allow for the mid body driving band, which is still pretty good. Still mostly a stupid idea since the Russians dropped it after the 45 mm. A standardised HE/AA round might be a better idea. 

17thfabn said:

Are you intending to have the above 4" gun as your main field artillery howitzer? Or would you develop the 25 pounder as the original time line?

The 25 pounder is one of the "do no harm" equipment items which is allowed to proceed (with maybe a hurry-up encouragement) because it was considered so successful at the time that it should not be messed with. In fact, the artillery seems to be one of the British success stories, not just in equipment but in organisation and tactics. Quite a contrast with tanks, for instance...

The idea of the 4" 40 pounder tank gun round (a necked-up 17 pdr case), just as with the 3" 20 pounder (necked up 57mm) is to maximise the HE effect. That requires relatively thin shell walls to maximise the capacity, and that in turn requires a low operating pressure and therefore a relatively low velocity. So the length of the shell might as well be maximised since only a small part of the case would be required for propellant.

Now we can debate how far this process should be taken. Most of the German 75 mm tank/AT guns fired the same 5.8 kg HE shell (with a 680 g fill) at around 550 m/s, much less than the AP rounds - even the L/70 fired it at only 700 m/s. So they seemed to follow a similar principle, without taking it quite so far. At the other extreme, the British and US 76mm HE tank gun shells initially fired the HE at the same velocity as the AP, resulting in an inadequate HE effect. 

Not quite the same principle, but the British HESH tank gun shells are also very thin-walled and high capacity, and fired at low velocity.