My original intent when first posting this thread, was to try and establish whether black powder substitutes such as Pyrodex or 777 typically used in muzzle loading revolvers and classed as propellants are subject to the same safety risks associated with air spaces as black powder loads when used with lighter loads in the same firearms.

I was also curious to find out if anybody had any real evidence that accuracy might be compromised if the projectile were to be seated some way below the mouth of the chamber which would be the case if a filler were not be used with a lighter load.

Although I have learned a lot from all those who have kindly taken the time to provide experienced input, I'm still not really sure if black powder substitutes can be treated in the same way as nitro powders in respect to the need for fillers or indeed if anybody has any real evidence of improved accuracy if the projectile is nearer to engaging the rifling when loading the chamber.

Any further feedback would be much appreciated.

Brian