Hi,
It was not powder fouling that doomed the Fergusons. It was the weakness in the stock behind the lock. All Fergusons that show hard use such as the 2 ordnance rifles still surviving and several made for the East India Company, are broken across the stock just in front of the screw plug. I've built 3 Fergusons and frequently shoot the one I own. Patrick Ferguson hints at a lube for the screw plug in some of his correspondence. I use a mix of beeswax and Crisco and am able to fire 30-40 rounds without cleaning the screw plug. A real weakness almost never mentioned is that the chamber opens for loading with one full turn of the trigger guard handle. A second full turn and the plug drops out of the gun and it is easy when loading the rifle to allow the handle to swing a second turn. That could be fatal in battle and to return the plug, you have to orient it correctly so closes with the handle pointed back. It is NOT a soldier friendly gun to shoot. Although there is a lot of hyperbole claimed for speed of loading the gun, Ferguson wrote that the real advantage was his riflemen could easily load lying down behind cover.
dave