Ruger warrants the Old Army to be safe with as much black powder as you can get in the chamber and still have room for a projectile.
I don't know what Ruger has to say about ersatz black powder, such as Hodgdon's 777 or Pyrodex. It would be best to check with the powder manufacturer on that point.
The Old Army is noted for being an extremely strong gun. Yet, it is not designed to use smokeless powder --- regardless what some knuckle-dragger tells you. And if anyone tells you different, ask them what ballistics lab was used to test pressures in their gun.
