Welcome to the Forum.
As you may know, all of these muzzleloaders are meant to be shot with patched lead balls.
The balls are smaller than the actual bore size to allow room for the patching material.
That said, the problem with a .67 or .69 caliber gun is the cost of the balls if you can find them.
Dixie Gunworks offers .600 diameter lead roundballs but the price is totally insane.
http://www.dixiegunworks.com/produ...=3662&osCsid=95693a3bf78c6b2f29b5693fb2debaee
The 2011 catalog showed these to be priced at 33.25/100 but they have jacked up the price to $50.50/100 plus postage at their website (link above).
In their catalog they say:
"Our lead balls and bullets are packaged and sold 100 per package
or can be purchased individually for $.50 each: however, you must purchase a minimum of four per each size ordered. (underline added).
They have undoubtedly raised the individual ball price but if you are interested you can call them at (731) 885-0700.
Because the balls are .070 undersize they should be patched with at least 2 layers of thick material like denim. Even then the accuracy will be very poor.
The powder load for something like this would be about 25 grains of blackpowder.
You cannot use one of the synthetic black powders in a flintlock for priming the pan or for the main charge without a LOT of mis-fires.
Real black powder can be difficult to find at most local gunstores and you
must ask for it. Call ahead to see if they even have it as most don't.