So how about this as a scenario: in the Desert campaign of 1941 both sides notice that the Italian AT guns at surprisingly effective at short ranges when using their EP shells, more than any HEAT effect would indicate, so for the Germans they test them out and notice the effect of the shell splatting on armor and start designing similar types of shells for use when encountering the T-34s and KVs of the Soviets, plus of course the Matilda tanks, as it enables their lower velocity weapons to knock them out. As such ammo become more common the Brits pick up on it and capture some in late 1941 during Operation Crusader. Testing of EP shells starts them on the path to developing their own in 1942. A problem with the shells is that they are relatively short ranged due to the lower velocities needed to work properly and given WW2 fuse sensitivity, so they aren't as useful in the Desert and in Russia, but are more useful than WW2 HEAT and let low velocity howitzers be even more effective against buildings, fortifications, and armor. They find their niche and let even smaller guns remain useful at closer ranges. In the 1942 campaign even the 50mm tank guns and short 75s can then deal with sloped armor Soviet tanks, while giving Assault Guns an enhanced impact in urban terrain without having to have massive cannons. They enable supporting armor to blast 'mouse holes' for infantry in addition to blasting down defensive positions. Perhaps in Stalingrad that would helped knock out major fortified buildings like Pavlov's House. Perhaps that then prevents the race to build 'super gun' AFVs that could blast away at long ranges against Allied armor, as even the standard PAK40 could knock out the heaviest of tanks within 1km where most tanks were knocked out. When the British field their own HESH it is initially only in response to the Tiger, because until 1943 they really don't have any enemy armor that can't be handled by existing guns (the 2 lber just needs to be replaced by the 6 pounder). HESH then appears for the Allies in the Sicilian Campaign for use against heavy German armor, but remains rare, and while it is somewhat of a rarity in German hands in the West because armor just isn't that heavy and a normal 75 can knock out all Wallied AFVs. Meanwhile in the East that is where it would likely have the biggest impact, as even the PAK38 (50mm) AT guns can handle T-34s; certain shots against heavy Soviet armor available even later in the war would still even be effect with 'just' a 50mm round (say one with reduced propellant and a longer projectile containing more RDX). Due to the shorter range of such shells though that would likely cause the Soviets to 'stand off' and engage from longer range with their heavier guns as they did IOTL. They'd have to be more cautious of closing due to AT gun ambush by smaller guns that could be more easily manhandled and concealed, but still be very deadly and have a higher rate of fire. Perhaps their use just leads to more artillery and longer range direct fire support weapons to neutralize any exposed defensive weapons in ambush, while forcing more recon unit use to make sure AFVs are a clear path. Defenders would have a significant advantage from ambush, but attackers would have a more effective demolition shell for use against defenders in fortified positions. Not sure if that cancels enough other out?