.357 Great Western Atomic was, as best I recall, standard length .357 Magnum brass, 158 gr bullet, and 16 grains of old Hercules 2400. That back when 15 gr was a good stout full charge magnum load. We are more cautious now, and the powder may not be the same, either.
Never heard of a Herter's Atomic.
Herter's First Rule of Firearms Safety: Never go hunting or shooting with somebody who might gain from you turning up accidentally dead.
Herter's Rule of Hunting Ethics: Shooting a gorilla is about as hard as shooting your neighbor in his back yard, and should be punished the same way.