I'd think thats is exactly what you do, stab them...to start.
Hyena runs up on you and he's in close thinking he has the drop.
Water Buffalo goes to his pocket and comes up with the brass knuckle knife gun. He stabs a bitch in the gut/ribs/chest/thigh and while doing so pulls the trigger firing shot directly at skin contact distance directly into the hide of the hyena.
It then would not matter that the cartridge has low power numbers and/or a small caliber because the wound alone from a contact injury is going to hurt a mother real bad even if it does not kill him. The point of this is to stop if not distract the hyena enough so that the water buffalo can either make a retreat or turn the tables and stab the same sucker another time or two possibly firing off more wound making rounds into the hyena up close and personal or crushing his nose and cracking his jaw with an open handed slap via the knuckle part of the weapons hilt/grip.
IMHO this is a pretty ingenious weapon Swiss Army style.