Does anyone know of good reference materials that discuss the above referenced guns and explains the performance differences between them? Was it simply the difference in calibers? Propellant weight/efficiency? Metallurgical differences? Other differences?
TIA
It is indeed mostly due to muzzle velocity. When you look at a gun it generally has a second number: 75/40. The second number is the length of the gun measured in gun calibers: e.g. 3000mm or 3m. The longer the barrel, the higher the muzzle velocity. However, you also have to consider metallurgy. Soviet metallurgy was notoriously bad and German metallurgy deteriorated toward the end of the war due to the loss of access to nickle and other raw materials needed to harden steel (which should, but does not, affect armor factors.)
Consider also muzzle brakes. Muzzle brakes redirect propellant gases and allow the installation of larger guns on the same size turret. The British 17lbr was the absolute max that you could install on a Sherman turret, allowed by the use of the muzzle brake.
Finally, some guns used squeeze bore principles to increase muzzle velocity. The German 28mm anti-tank rifle actually had a 20mm penetrator with a mesh which squeezed down to 20mm by the time it is released from the bore, increasing muzzle velocity and hence, penetration. However, such high velocity for a small penetrator led to the shattering of the projectile when it hit, so special manufacturing using nickle projectiles had to be created.