But, I'm placing a thin "overshot" card over the powder (to keep the lube away from the powder) then a lubed felt wad, then ball, all held in with 2 "overshot" cards.
The theory I've heard is that the soft flexible wadding behind the ball somewhat cups it and keeps it centered in the bore.
YES - this "theory" does work and is the method I use in a SXS 12 bore shotgun shooting ball. I actually use a cup-shaped wad under the undersized ball, to hold it in the middle of the bore. With this style of loading, I was able to smack a 10" square plate of steel shot after shot, offhand at 100 meters. Thus, it works.
The only problem in a ML, is cleanliness of shooting, and not having the fouling build up in the bore, shot after shot. 2soft lubed wads might bear experimentation if someone is interested in finding out the MOST accurate load. Know also, the further you can project the ball BEFORE it takes on a spin and turns into a curve ball, the better. This simply takes MORE powder - and experimentation. WHAT FUN!
More power & ACCURACY can be realized with the largest ball that will fit inside the bore, still using the method of the soft-cup wad. That is, a .685" ball will likely be more accurate than any smaller ball due to the decreased "windage."
Lee sells a .690" ball which might be perfect as-is, if casting, say .001 or .002" smaller than the bore size.
Lead (PB) as pure as possible, will cast the smallest size the mould will produce.