I previously shared how the PzIII did in Soviet hands. In summary, the 30 mm plate proved far too brittle under fire from a 45 mm gun, shattering into pieces from just one hit. Curiously, British trials find the exact same thing.
Under attack from 2-pounder AP shot, the first projectile chips off the edge, but the second shatters the plate and causes severe cracks. When the remaining pieces are tested, they too crack into pieces after one or two shots with APC shells at 20 and 30 degrees. The British had more than just one hatch, however, and also observed the welding seams coming apart after the shots. The velocities matched those for 1200 yards in the test with AP and 1000 yards with APC.
However, things get even more interesting after. In trials against a later model PzIII, one with 50 mm of front armour, the 2-pounder can get a shot through the front plate from 100 yards, although the projectile shatters. Interestingly enough, in Soviet trials, the 2-pounder only managed to penetrate the upper edge of the StuG's upper plate (also 50 mm thick) once from 100 meters. The testers were unable to reproduce this penetration from 50 or 100 meters. The Soviet conclusion is that the 2-pounder cannot penetrate 50 mm of armour at any distance, whereas the British are content with its ability to penetrate the German tank from 100 yards. The British also establish that the 2-pounder can defeat a PzIII from the front from 300 yards with HV ammo, but the Soviets didn't have any available.