After 3 days of Sweet's solvent in a dry barrel,
I could see a small area about 1/8th inch round probably light surface rust forming.
The front brass bead goes right through to the bore and was showing some green colour.
The rest of the bore looked as original, (using a magnifying glass.)

Cleaned it out, re-oiled, no sign of etching or pitting, so I won't be worried of leaving Sweet's in a barrel for a shade longer than 15 minutes.