(Yawn) someone called? 
This is from my website's updates page for my book "Rapid Fire":
The smallest calibre HEAT round I know of in WW2 was one for the Japanese 57mm low-velocity tank gun (yes, that's 57mm not 75mm).
Whether a 40mm round would have worked very well I doubt. Early HEAT were nowhere near as effective as present day ones (a HEDP - combined HEAT and Frag - round for a moden 40mm grenade launcher can penetrate around 50mm armour).
This is from my website's updates page for my book "Rapid Fire":
"Automatic grenade launchers were developed in the USSR in the 1930s. Taubin, who led the OKB-16 design group, became obsessed with the idea of an infantry support weapon that could fire fragmentation rounds in both direct and indirect fire. By 1935 he had found support in the top ranks of Red Army, and began development of the 40.6mm AGL; the projectiles were based on the 40.6mm Diakonov rifle grenade, the weapon itself was long-recoil, locked breech gun with top-feed five-round chute or box magazine, usually mounted on Maxim-type wheeled mount. The cyclic rate was 200 rpm. Muzzle velocity was 120 m/s and the maximum range 1,200m. It weighed 16 kg (24 kg with sight) and one round of ammunition weighed 0.6 kg. It was proposed as infantry Company-level support weapon, as a replacement for 50/60mm mortars, as well as a mounted weapon for riverine armored vessels of Amur river fleet. By 1938 small-scale manufacture of the 40.6mm Taubin AGL was initiated, and larger caliber weapons of same concept were in development for vehicle applications. A few 40.6 Taubin AGL's were used during the Winter War with Finland in 1940, apparently sucessfully. However, changes in the top ranks of the Red Army and especially the arrest of Marshal Tukhachevsky withdrew support from the AGL concept and Taubin was destroyed by rivals from the Mortar school - he was arrested and executed in 1941."
The smallest calibre HEAT round I know of in WW2 was one for the Japanese 57mm low-velocity tank gun (yes, that's 57mm not 75mm).
Whether a 40mm round would have worked very well I doubt. Early HEAT were nowhere near as effective as present day ones (a HEDP - combined HEAT and Frag - round for a moden 40mm grenade launcher can penetrate around 50mm armour).