The culprit is an Armi Sport 3-Band Richmond. After firing 40 shots, brushing after every 10 or so, there is so much hard fouling in the channel that I have to use a pick to break it up. Before firing, I swab all of the oil out of the bore, and use pipe cleaners and Q-tips to clean the flash channel. I am shooting 50 grains of Goex 2F and a Hodgdon 416 grain Minie. None of my other muskets do this. My question, I guess, is what causes this and how to avoid it?