There is a vast literature on the tanks of World War II and their armament. Two things mystify me. For the first half of the war, most British tanks were armed with two-pounder guns. These were 40 mm cannon that fired an armor-piercing shell that was effective against all German tanks at the beginning of the war, and pretty effective even after the Germans improved their tanks and armor in 1942. But in North Africa, British tanks were ambushed time and again, hit by antitank guns, including the famous 88 mm gun. They had no effective reply, because, the books say, “the two-pounder lacked a high-explosive round.” Why was this true? Other 40 mm guns at the time—such as the famous Bofors—fired nothing else. Was there something special about the two-pounder that precluded a HE shell?
In September 1942, just before El Alamein, American-made M-4 Sherman tanks arrived in North Africa. They had a 75 mm medium-velocity gun that fired a very good HE shell. I have read that it contained 1.5 pounds of explosive. It was excellent against soft targets such as antitank gun crews. But by June 1944, when M-4s landed in Normandy, the 75 mm gun’s armor-piercing shells could not penetrate the front armor of the new German Mark 5 Panther. Even the newly introduced 76 mm gun on later model M-4s had trouble with both the Panther and Tiger’s armor, though its AP rounds were considerably better than those fired by the 75 mm gun. I have recently read that many American tank units kept at least a few older Shermans, with their 75 mm guns, in service till the end of the war, because the so-called 76 mm gun (it was actually a 75, too) fired an HE round with only one pound of explosive, much less effective against soft targets.
What was there about the two-pounder that made it impossible to create an HE shell? Why did the American 76 mm HE round contain less explosive than the 75 mm did?