Filler Question.

[Turnup]

Its the saltpetre (Potassium Nitrate) that provides the oxygen for blackpowder to burn.
Blackpowder is an explosive. It detonates and will do this outside a gun chamber.
If there is an air gap the powder gases start to expand. If they hit a solid such as a ball the pressure peaks at that point which causes the gun to burst. Without an air gap there is a progressive expansion of the gases that push the projectile out.

The peak pressure hitting a solid or obstruction also happens with smokeless. Several years ago there was what was termed a "Rash of Pimples" appearing on shotgun barrels.
It was eventually discovered that clay shooters who used the first barrel and did not fire the second barrel for a while was causing the crimp to weaken, due to the recoil, and allow the odd pellet to drop into the bore.
When they did fire the second barrel the charge hitting the loose pellet in the barrel caused the pressure to peak momentarily at that point and cause a tiny pellet sized bulge on the bottom of the barrel.
A larger obstruction would cause the pressure to peak higher and bulge or burst the barrel.

Agree with this mostly. Air in a BP load plays no part in the chemical reaction - BP has all the oxygen it needs already chemically bound in the Potassium Nitrate (which is released when heated and there is exactly enough oxygen to combine with the Carbon and Sulphur completely). The presence of an air gap changes the propagation of the deflagration reaction within the powder in unpredictable ways because the BP grains are not initially constrained to remain in one place. Sometimes you get away with it (I have fired a BP revolver where I had omitted the filler so the BP was loose - competition event and had no time to sort it - no harm done this time) and sometimes you don't. It does seem to be more of an issue with pistols than revolvers but this is not science.

Disagree on BP detonation. Detonation is defined as a deflagration (burning) reaction which proceeds through the deflagrating material supersonically. There is no known way to detonate smokeless. There is some suspicion that it is just about possible to detonate BP, but if it occurs at all it requires extreme conditions of a kind not encountered in firearms and even then the result is deflagration which is only marginally supersonic. Note that here I an referring to the speed of propagation of the reaction in relation to the speed of sound in the medium (in this case BP), and this is neither related to the speed of sound in air or the speed of the resulting projectile or combustion gasses.

BP requires some form of containment to produce a bang when ignited - free burning rate is not fast enough to go bang. A large amount of BP in a loose pile can self confine but it takes about a pound of the stuff to produce a bang rather than a "wuff". Small confinement (such as loose in an open ended cylinder) can produce a bang with much less material.

For these reasons BP is classed as a low explosive.

High explosives detonate well supersonically and produce characteristic shattering in metals rather than bending/stretching/tearing such as caused simply by overpressure.

It is my experience that:

a) reduced loads in revolvers can increase accuracy = this is analogous to the holy grail sweet spot Mv sought by many target shooters (air, rifle, and pistol/revolver).

b) seating the ball close to the chamber mouth is most likely to give the best results.

c) I do believe that there is also a sweet spot (or possible more than one) when seating the ball below the chamber mouth, at least in my ROA, but with no filler there are two variables (adjusting a BP only load varies seating depth and the resulting Mv) so it is difficult to experiment with any good control