1. Filler...I've read of the use of filler such as cereal, cotton, etc to fill the gap between the top of the powder and the base of the bullet. Does this material then prevent the pressure problem that would otherwise occur if that space were empty? It's not clear to me how the filler is different from an air gap.
2. If there is an air gap that has to be filled, does the wad that is placed on top of the powder have the purpose of preventing the powder from 'slumping' when the cartridge is horizontal.
3. If slumping occurs, what is the effect when the powder ignites?
Here is an analogy that has come to mind when trying to understand this problem. I imagine a locomotive trying to push a heavy load from a standstill. In one case, the locomotive is attached to the load, and the other is when the locomotive is separated from the load by some distance. In the first case, the locomotive transfers its power directly to the load when the throttle is advanced...and in the other, the locomotive accelerates rapidly and then rams into the load with predictable results...the load no longer is a load but becomes an obstruction.
Does this illustrate the problem in a reasonable way?