If you can't find the Rusty Duck, you may want to try a bottle of Hornady's "One Shot Muzzleloader Cleaner & Protectant".
I'm pretty much a soap and water guy and hardly ever buy a special cleaning product. But I recently ran across an $0.88 clearance sale for the One Shot stuff and picked up a couple of bottles. I have to admit - it's pretty effective (but pretty expensive at regular price). It has a slightly oily feel to it. I've found that it dissolves and neutralizes black powder fouling very effectively. A patch or two down the bore and a spray on the breech followed by whisking off with a small paint brush will keep everything safe for a day or two if I can't clean the gun right at the end of a range session.
So I still do my routine and thorough cleaning with soap and water, and save the One Shot for those times when I want to delay full clean-up until the next day.