Given the way in which tank guns were actually used, the main priority for our alternative weapon would be to greatly enhance the effectiveness of HE shells while at least matching the 2 pdr's AP performance, and at the same time keeping the gun and ammunition small enough to fit into the restricted British tank turrets of the period. The size limitation would rule out a medium-velocity 75+mm gun, so I have chosen as a starting point the 57x307R cartridge of the 6 pdr 8 cwt gun – a 19^th century naval gun which had been fitted to early WW1 tanks. This cartridge was the same length as the 3 pdr (47mm) tank gun round used in the 1920s Vickers Medium tank and only slightly longer than the 2 pdr (40mm). It was significantly shorter and slimmer than the 57x441R case of the 6 pdr 7 cwt tank/anti-tank gun which entered service in autumn 1942. These rounds can be compared in the photo on the right.

The 6 pdr 8 cwt had a modest performance, driving a 2.72 kg shell at 540 m/s from its L/40 barrel to give 400 kJ muzzle energy (the later, cut-down WW1 tank guns had L/23 barrels and an even lower velocity). However, by the 1930s metallurgy had improved greatly from that of the 19^th century, as shown by the fact that the 6 pdr 7 cwt gun weighed 1 cwt (50 kg) less than the older gun despite having a slightly longer barrel and a muzzle velocity of 820 m/s, developing 940 kJ energy (and obviously a far higher chamber pressure). So how much energy could have been squeezed from the 57x307R case if fired from a 1930s-designed gun?

The nearest comparator is the German 5cm L/42 KwK 38 "short" tank gun introduced in the Pz III, which used a 50x289R case about the same size as the 57x307R (see comparative photo on the left). The KwK 38 developed 565 kJ muzzle energy. The larger calibre of the 6 pdr would have obtained greater efficiency from the propellant, enabling our new 57x307R loading to perform rather better – probably around 645 kJ. 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, but the shorter chamber compared with the 57x441R would have resulted in a slightly longer bore.

So that's the basic parameters sorted, now lets turn to the new ammo. A muzzle energy of 645 kJ would punt the standard 2.72 kg shell out at 690 m/s (for comparison, the German 5cm L/42 fired a 2.04 kg shell at 745 m/s). However, I would want to adopt the approach the Russians took with their 1930s 45mm tank/AT gun, and choose an HE shell which is much heavier than the AP type, and fired at a lower muzzle velocity. The new gun would be able to fire the old naval ammo for low-cost training (and in combat, in an emergency) so it makes sense for the new HE loading to match the original trajectory. That means a muzzle velocity of 540 m/s, which for 645 kJ energy would mean a shell weight of 4.42 kg (9.7 lbs). However, this is probably a bit high since such a long shell would penetrate deeper into the case, reducing the space for propellant, so we'll call it around 4 kg; that's a 50% increase in the shell weight and effectiveness over the standard 6 pdr, and a 360% increase over the 2 pdr. The shape of the shell would be fiddled with to match the ballistic coefficient of the naval shell, so the same sight settings could be used. The nose fuze would be settable to instant action (for fragmentation effect against troops) or delay (for blast effect within buildings and vehicles).

Now to the AP ammo. Firing the standard 2.72 kg shot at 690 m/s would deliver respectable penetration by early-WW2 standards, but a rather curved trajectory. On the other hand, APDS was insufficiently advanced at that time to be reliable. So I'd go with APCR (HVAP to Americans) in two versions: Mk I with a hardened steel core, and Mk II with a tungsten carbide core. The two versions would be designed to have the same weight, muzzle velocity and external ballistics, the main difference being that the penetrating core of the Mk II would be slimmer to keep the weight the same, offering something like a 50% increase in penetration over the Mk I. The diameter of the tungsten core of the 6 pdr 7 cwt APCR was 28mm, meaning that a steel-cored version would measure around 40mm.