When you are dealing with smokeless it is an entirely different game. Smokeless pressures can spike. You would need to think of the total mass.
I would also think you don't want the balls to impacting each other in the bore. Might be creating a temporary bore obstruction.
Blackpowder used buck and ball, no problem, however, blackpowder is very low pressure.
If I'm not shooting, I'm reloading.