Triple, Now I catch your drift, you're using the sabots, and the copper jacketed bullets, and that other modern stuff, poly, etc.. My BP rifles get only pure lead, the balls are patched, and the conicals are well lubed. I admit, I did try the sabots in my .54, and ended up using conventional methods to get the melted plastic out, a real pain.
I shoot a lot of CAS, using BP exclusively, and was using wheel weights for cast bullets; I noted a lot of leading, to the point where I consulted a couple of gunsmiths. Even had one open the forcing cones on my pistols, as I was too bullheaded to listen to what they told me, "the lead was too hard". After I started using pure lead, the leading went away, I know it don't make much sense, but that's the way it is.
I agree soap and water won't get the lead out, but it does break down the fouling. As fredj said, just plain old water will break down the fouling, the soap, like the window cleaner, and other concoctions seem to speed up the cleaning process some.
I believe from my 30+ years of messing with the holy black, the key to keeping the arm in good shape, is after cleaning it, it's important to make sure it is thoroughly dry, then grease 'er down till next time. Granted if you ran some modern solvent on a patch down one of my BP arms, I'm sure it wouldn't come out lily white, but try the same thing with a smokeless arm and you'll find the same thing after brushing with a bronze brush. What ya won't find is rust on any of my guns. Didn't mean to get so windy.
RR