It's not my assumption, but a well know fact, that the heavy load you said you used in taking that fox is a dangerous load in the thin wall chamber of the Brown Bess with the standard barrel.
I am first to say that I am no expert and someone here probably knows the chamber pressure generated (extremely high) from loads like you used. I've proof tested barrels with loads like that and did split the barrel and destroyed a nice old percussion double barrel shotgun with a double charged proof load, as I know others might have experienced with proofing a barrel or checking a antique for safety.
Not commenting on your post to put you down by any means, but to encourage you and others about such over-kill loading.
Rick
