I just picked up one of these up at a estate auction in a .54 caliber. I have 2 .50 cal in lines and 2 .50 cal flintlock but this is my first .54 cal. What should I start out with in a bullet and charge? Any help is appreciated..

I don't have one and never had one. But with that gun's 1:38" rate of twist I think I would try the 425 grain Hornady Great Plains bullet over something like 70 or 80 grains of Pyrodex RS or GOEX.

348 gr Copper Aerotip Powerbelt with 80-90 grains. (page 1)
435 gr T/C MaxiBalls or Maxi Hunters (page 3)

I'm assuming that you won't be shooting sabots, since you already have two 50-cals.

http://www.ableammo.com/catalog/blac...03.html?page=1

The .54 caliber Black Mountain magnum shoots patched round balls surprisingly well. Start with 70 grains of powder.

I had a TC FireHawk in .54 with the 1:38 twist it was a good conical shooter after I sent it off to have a QLA installed it was a sabot shooter...
My hunting load was a Knight lead 260 gr bullet in a red sabot and 100 gr. Pyrodex P. Put a lot of deer in the freezer with that load.

I have a T/C Black Mountain Magnum but in .50 caliber. Mine has a 1-28 twist I was told. I also have mine scoped.

I found that 90 grains of 2f black powder and a Hornady 385 grain conical is very accurate. Also mine does shoot a number of different sabot combinations. But when I try to increase the powder charge... after all it is a magnum, well the accuracy really disappears.