The question of cast vs rolled armour is often encountered in specialist literature. It is typically accepted that cast armour needs to be about 10% thicker to provide the same resistance as rolled armour. This was the case with T-34 turrets: cast ones had a nominal thickness of 52 mm, turrets welded from rolled armour had a thickness of 45 mm. However, trials were held in late 1940 to confirm this theory. Defective hulls and turrets rejected by QA were used.
Results of trials with different types of shells were compared to determine the limit of partial penetration (PTP) and limit of complete penetration (PSP) in meters per second.
Considering that the muzzle velocity of the model 1937 45 mm gun is only about 750 m/s (depending on the round), the gun can only penetrate either the cast or welded turret from nearly point blank range. The sharp tipped round fared a little better, but the penetration range for both types of turret is comparable. The 76 mm gun used in this test was the model 1902/30 gun with a muzzle velocity of 650 m/s. Penetration was definitely possible, but also at close range. Note that the casts turrets are actually better at resisting this gun than the welded turrets.
In general, the commission came to the conclusion that the two types of turrets are generally equivalent in resistance to shell fire.