Best product for cleaning black powder fouling is plain, tepid tap water.
Many prefer complex formulae of just about everything.
I think all (most, at least) agree about the worst thing to put down your bore is Salt water.
Cap lock guns get some salt from priming compounds and at least one replacement for black powder is positively unspeakable for rusting your guns
Whoops . . . commercial soap (ie., P&G stuff) ALL contains salt. Yeah, salt, as in Sodium Chloride, NaCl. It has been made that way for the last half century. At the time was part of reducing phosphorus in lakes & streams. Huh? Yeah, that is a long story you don't care about.
Commercial stuff which is called soap gets some excess of lye (sodium hydroxide) in production. So that it doesn't act like Grandma's lye soap, this excess is neutralized by a little hydrochloric acid. This turns that sodium hydroxide into sodium chloride. Salt. The stuff on potato chips. The stuff most Northern states put on icy roads in winter. The same stuff that rusts out your car.
Clean with distilled Yak fat from Uzbekistan if you prefer it to plain water, but it really is best not to add soap.