If I recall my Ruark et al correctly, elephant skull isn't tough to penetrate because it is -hard- and thick, it is tough to penetrate because it is "spongy" and thick. More like modern tank armor, it doesn't shatter, it absorbs the energy, thus the need to punch a solid all the way through rather than just create a breach.
Penetration wasn't so much the problem as was actually
hitting the brain, a (roughly) football-sized mass in a very large skull. A near-miss doesn't do much good, since the spongy bone won't transmit shock very well to the brain. John "Pondoro" Taylor based his "Knock Out Values" on his opinion of how effective an elephant gun would be should the brain be missed by only a bit; he favored large cartridges, and opined a near-miss of the brain from a .600 NE could knock out an elephant for, IIRC, a half hour or so.
Of course, this being the 21st century, it's unlikely anyone will ever field test Pondoro's claims in a scientific manner.
Speaking of brain shots . . . how many people know
exactly where to shoot in order to hit a walnut-shaped brain in a four foot T-rex skull? Suspect even Bell would find it a challenge!
