My modern setting has these humanoids called the Degenerates whose culture draws inspiration from those of Indo-Iranian steppe nomads like the Scythians, Sarmatians and Parthians who fight as mounted riflemen that ride mutated animals donning bulletproof armour. The Degenerates' most skilled warriors are armoured troops called cataphracts wielding heavy weaponry who sometimes ride dinosaurs dubbed Ekhidnades similar in dimensions and looks to the tyrannosaurid Albertosaurus sarcophagus (save for the fact that they have osteoderms, spiny protofeathers, stronger jaws and can expel streams of combustible liquid from their mouths).
I've envisioned that a cataphract would ride their Ekhidnades using a three-person saddle, sitting in the front most seat near the Ekhidnades' neck with while firing a minigun (like a GAU-19 or M134) anchoring onto a swivel mount fixed to the saddle. Said minigun hooks up to car batteries and ammo crates which would provide power and ammunition, somewhere around the Ekhidnades' flanks. Two other cataphracts, would meanwhile, ride shotgun and provide additional firepower with light machine guns, anti-materiel rifles or RPGs as the Ekhidnades rampages about, biting, trampling or immolating anyone unlucky enough to get in its way.
Reference image for the Ekhidnades' armour
Would an saddle mounted mini-gun be useful/practical on such an animal?
Why or why not?