Go slow - good luck. We unbreeched a Manton 14 bore double shotgun - of the 1840/1850 period after soaking in a solvent I found in my Dad's shop supplies. That solivent or penetrating oil was a green colour. I don't know what it is, but smells like Varsol and Kerosene with perhaps some mineral oil. I don't knwo where the 'green' colour comes from. That is a combination that would allow 'soaking' from the inside.

Kroil is not available here - must be picked up State's side of the border. I am rather extravagant with it, even to giving my bro 1/2 of what I had. I mixed up some more, 50/50 with Butches Bore Shine as a solvent for my .17 centre fires. It really works well with no need to neutralize what's left in the bore, after cleaning as should to be done with all 'copper' solvents.