Well, the six-pounder [57mm gun] versions seem a bit undergunned compared to the Shermans' 75 and 76mm shooters.
The 57 mm had slightly better armor piercing capability than the Sherman 75. The 76mm was better. Sources differ about how many and how fast the 76mm Shermans came online. I read somewhere that @ 1/3 of Shermans were 76mm by the time of the Ardennes offensive.
The armor on the Churchll would keep out fire from the Mk IV tank in the frontal arc at most ranges. Not so the Sherman.
Walt
The 57mm/2¼-inch shot round would penetrate a little better than the equivalent 75mm [3-inch] APC [Armor-piercing, capped] solid projectile, due to having similar velocity, but the 57mm projo having a little better shape and lesser frontal area. But the 76mm HVAP or *hypershot* ammunition was the real tank killer, especially in the Buick M18 Tank Destroyer, in which the new ammunition was first field tested.