UPDATE... I finally had a chance to try out the smaller .501 Thor bullets today. I also bought some T7 primers to see if they would help with the crud ring. All I can say is WOW! These are the most accurate bullets I've ever shot out of a muzzleloader! I shot 4 times at 50 yds, then 3 times at 75 yds. Made an adjustment and shot twice more at 75. All but one of my 50 yd shots were touching and all three of my first 75 yd shots were touching. In fact they looked like one big hole! With open sites, I've never had a group this good.
The new T7 primers did seem to leave less of a crud ring than my other primers but didn't eliminate it completely. The good thing is I was able to shoot, then re-load with a fouled barrel and shoot again. I was able to do this with all my shots, one on a clean barrel, then one with a fouled barrel. They weren't the easiest to load but I feel confident that I could re-load in the field okay.
I'd still like to do some more testing and use some BH209 but with hunting season only a couple weeks away I'm pretty confident with the load I have worked up with 90 grains of 777. I'm planning to pitch the T/C after this season anyway and get a new Knight, so I'll be back to starting all over anyway.