Unburned powder

[Kensal Rise]

An oversize bore could contribute. This is not uncommon in old guns that have honed bores. A larger bore allows gas blow-by, reduces burn confinement, and leaves residue... especially with some powders like Red Dot or Clay Dot.

[Bill Murphy]

John's answer would be my choice. RST low pressure loads, together with the large bores, (.750 on some Parkers), contributes to less complete burning of the powder coating. I doubt that the incomplete burning extends to the explosive components of the powder. Call Alex and ask.