I'm planning to start with AA 5744 in the 45-70.

Western Powders states to NOT use fillers or wads. They have me convinced.

They also make the claim that one reason AA 5744 works well in the old BP cases is because of it's low density but that would appear to be simply false. Looking at the Lee density table that comes with their volumetric powder measure 5744 has typical density for a stick type powder. A double charge would fit, and that would lead to disaster.

"The price of good shotgunnery is constant practice" - Fred Kimble