Alox mate, it covers a broad range of velocities. I use it on my .357mags and my .44 m/l get no fouling in either.
Your fouling is possibly down to using BP, could it be mixing with the lube while hot and cooling as a goop? If so maybe beeswax/lard or trex would help but I doubt it. Having just read what you are firing, I'd have said alox was best at the breech pressure and velocity that heavy slug is running at.
“If a cricketer, for instance, suddenly decided to go into a school and batter a lot of people to death with a cricket bat, which he could do very easily, I mean, are you going to ban cricket bats?” :- Prince Philip said after Dunblane