The Browning Mountain Rifle was supposed to be an excellent rifle. I have never seen one, but have read of people that own them and claim they are a great shooter.
Being a .54 caliber I would start it out with 80 grains of 2f powder and a patched .530 roundball and see what happens. Keep us posted and if you can, throw a picture up of it. The Browning Mountain Rifle was always renowned to have beautiful wood stocks on them and nice hardware to wood fit...
Also let me take this advantage to welcome you to the forum. It's great to have you here....