[B.L.E.]

Think of duplex loads as ignition augentation

If anything, I feel it's the smokeless that needs a kick from black powder to get burning, not vice versa. In large artillary, such as the 16" naval guns, a 2 kilogram bag of black powder goes between the 600 pounds of smokeless powder and the primer to kick off the smokeless charge if I remember correctly. The black powder charge acts as a booster for the primer.