I've only hunted a few times with black powder. For the first times, the only rifle I had was a breechloading .54 Gallagher Carbine. The brass only holds 45 grains powder, and someone advised me to use harder lead balls in order to maximize penetration of that "big and slow-moving ball".
Shot was behind and a little below the shoulder, and penetrated alright - the exit hole was not much bigger than the entrance. From that alone, I decided that soft lead was the best choice.
Next season, the same 40-50 yard shot, ball thru shoulder blade, flattened out to almost the size of a quarter.
First shot with the thru and thru alloy bullet gave me about 100 yard chase before dropping. The 2nd year using the soft lead ball that flattened out dropped in 2 steps.
Had either the alloy or soft lead balls with that comparatively light charge failed to perform as well as they did, then maybe I'd better understand why some folks find it necessary to require use of heavier slugs propelled by heavier charges (not a criticism of anyone in this thread).