Normal loads for 10 bore shotguns were 4 and up to 4 1/2 dams. They were not using heavy shot charges, though, but in the 1 1/4 to 1 3/8oz.

5 drams = 136.5gr. weight. What bullet or ball is to be used in this 10 bore, for water buffalo?

I would think it would work, but I personally would prefer more powder as designed into the guns. Should it not depend on how the gun regulates?

I think the 5 dams load vel. of little more than 1,250fps with round ball & maybe 1,000fps with a slug, will make for a steep trajectory, and unless a hardened ball is used, poor penetration.
It should work, but??

Greener's book lists 10 drams (273gr.) as normal 10 bore load with a hardened 674gr. round ball, I assume. The vel. listed for such a load is 1,600fps in an 11 pound gun.
My chart lists a 10 bore ball of .774" at weighing 700gr. in pure lead.