Filler Question.

[enfield2band]

You cannot really compare BP & substitutes to smokeless powder.
The firearms designed to use smokeless are far far stronger than any of the BR firearms on the market. Smokeless burns differently with a lower initial spike but a more prolonged pressure build up resulting in a higher overall chamber pressure.

You only need to look on YouTube and you'll find plenty of examples of people in the US showing what happens when you load a BP firearm with smokeless. It's mortally dangerous.
https://www.youtube.com/watch?v=Q_XNWeMqCT0

By the sound of the earlier replies the only people not using a filler ram the ball all the way onto the power anyway, regardless of the charge size.

And I'm with Tac 100%, Never ever load with an air gap for BP. I dont know how the physics of substitutes like Pyrodex & 777 work, but BP deflagrates (burns bloody fast) at a constant rate with oxygen present.
The air trapped in-between the grains is sufficient for the powder burn to build pressure to propel the projectile - However, if given more air the initial burn will last longer, this is where the term pressure spike comes from. BP builds more pressure than smokeless initially, but due to lack of air the pressure is only for a brief instant then the burn slows. The danger with the air gap is it provides enough air for the initial pressure spike to significantly increase to the point the firearm fails.

Edit - Good article here on longer air gaps cause by short loading the projectile for those who shoot long arms.
http://www.brimstonepistoleros.com/articles/squib.html

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.