I clean BP firearms with soap and hot water, as well. It's more effort than some other methods, but I think it does the best job.
That Weatherby makes me want to break out my photos from last summer.
Not firearms, but...
I picked up a bunch of reloading tools, dies, two presses, two electric routers, and a progressive shotshell press at an auction.
They had been sitting, untouched, in the corner of an open-sided horse shed for about 25 years, and were absolutely CAKED in disgusting grime. The routers were so bad that the rotors and stators were stuck together.
Solvents wouldn't touch it.
So, I soaked everything (including the electric routers) in a 20 gallon basin full of hot soapy water for about 4 hours - adding 3 gallons of hot water and more soap until overflow, every 30-45 minutes - and then alternated soaking, scrubbing with nylon brushes, sloshing things around with a broom, and (cold) high pressure wash ... until I lost sunlight.
The routers were blown out and left to dry for about 2 months, occasionally spinning the shafts, before testing (they worked just fine). And everything else was stripped down, scrubbed, and dried by hand.
I had a little bit of surface rust on a few things that were raw steel. But everything else turned out just fine.
...And for all I know, the rust could have been under the greasy, grimy horse-crap residue, to begin with.