It is impossible to shoot hardened conicals in traditional hunting arms with any accuracy.
The soft conical will not penetrate as well as a hardened RB. They also tend to veer off track when striking the animal. RBs generally do not.
This is why hardened RBs were used "back in the day" rather than conicals for the larger dangerous game. Lions and such did not need that much penetration.
The conical was tried in the 19th century and found to be unsatisfactory on several counts. The hardened RB was generally used for dangerous game until the advent of the BL guns that COULD use hardened conicals. Then the calibers were also reduced.
With good shot placement and an adequate charge of powder 6-7 drams of FFG it should work OK with a hard RB.
Pondoro Taylor was killing African Elephant and Rhino with a 10 bore using RBs in the 1930s when his ammo shipment was misdirected or lost. If a 10 bore with 6 drams and a hard ball will kill elephant I cannot see a 72 failing on Buffalo.
Its in his book "Pondoro" by John Taylor.

Dan