Doug - I seem to recall it was Turner Kirkland who used an original 4 bore Rawbone (Lyman's first edition of the muzzlelaoding handbook) for an elephant. He used a mere 300gr. of powder, and a 7 bore ball at 1,030gr. He had a 6 bore made in Belgium as a backup. The 6 bore shot a 725gr. ball (10 bore size@ .775=700gr.), which is very much undersized for an 8 bore, let alone a 6bore. For example, an 8 bore pure lead bal weighs 875gr., due to it's .835" size, whereas a 6 bore ball weighs 1,166gr. @ .919".
The penetration was as you stated, Doug - 20" and only got one lung as they flattened out on the hide. He used 11 drams, while 16 was the "Full Monty" for a 4 bore & with a larger ball.
Val used a very heavily loaded Navy Arms "Hawken Hurrican" with a 610gr. slug + 175gr. 3F - not for the faint of heart.
The pure lead balls lacked penetration on heaily skinned game, which is why hunters of 'old' used hardened lead, with addition of tin or murcury. I've shot both, of course, but suggest you stay away from pouring murcury into your lead pot.
Oh yeah - Baker's "Baby" shot a 3oz. belted ball. He had a 4 oz. conical made for it, which lead him into "Such scrapes" as he "eventualy gave up the conical as useless". A 3oz.(1,3125gr.) perfectly spherical ball is a wee bit smaller than 5 bore at 1,400gr., so yes, Baker's "Baby' could well have been a 4 bore as well, but not a 2 as reported by some 'modern' writers.