The water does a fine job of dissolving the black powder residue. What the soap is needed for is to dissolve the oil residuals. Water alone seems to leave a bit of the burnt oil in the bore. Once the gun is clean, the burnt oils show up as a dark brown or rusty colored patch even after using a good rust preventative lubricant. Its easy and cheap to add a few drops of dish washing soap to the cleaning bath ad you clean both the black powder fouling and burnt oils in the barrel.