I have done a few things along these lines and will pass on a few stories on some of the projects from not long ago.
I bought a real nice Type 99 with all the stuff still on it. Like your friend it belonged to an old guy, and he had it sitting out by the fireplace for decades. The wood was REALLY dried out and the entire bore was full of dust bunnies and fairly rusty. On this one after I got a cleaning rod to run through it and most of the loose crap out of the bore I got two bore snakes one soaked to the gills with oil, the other with solvent. Ran the solvent snake into it and let it sit over night, then go back and clean normal. Took a day or two going back and forth between the oil and solvent always checking the bore, cleaned up nice.
The one I am working on now is not rusty but just filthy. It is a mosin that I bought in 2019 but never did anything with. Checked it out last weekend to make sure it would not hurt me, and found it to be fairly dirty....make that really dirty. I shot it a total of 5 times and took it back to the shop for cleaning. Lots of scrub lots of patches and no real result, patches come out black for a while, and the bore cam shows it just as cruddy as last time.
With this one I clamped the rifle in the vice, rotated it on her so her nose is pointing down. Then take the cap from a bottle of water and fill it with modeling clay. Stick that on the muzzle to plug it. Now pour the solvent of your choice down the breach and stop when the bore gets full. Let soak. This has worked real well for me in the past, I usually buy old guns and they are always dirty. After that you can pull the clay and dump the solvent, or capture it in another container, you can re use it a time or two IMHO.
That is what I did.