Dear Mr Tall - you don't say if your new rifle is percussion or flint.

If it is a flinter, then around 35gr of FFFGg will be a good way to a start - go lower and work up. This can also be used for priming, of course.

This for Black Powder only. You cannot use a substitute in a flinter with any degree of success - even the priming pan must be charged with real BP - not a substitute.

If your rifle is a percussion, then around 10% less of the same of the same charge - working up your best accuracy.

Any of the usual BP subs, Cleanbore, 777 or Pyrodex, are measured by volume, not weight, but approximate to about 80% of the usual BP load - so your load should be around 28gr of the substitute.

Make sure you are using HOT caps.

I assume you know all about patching and so on?

tac