.loaded with 6, 7, and 8 grams of the same rifle powder....specs called for .97 grams, but that
Not sure what you mean by black rifle powder. There is no BP that is specifically for rifles. The two granulations commonly used in both rifles and pistols are FFg and FFFg.
Wikipedia is not a reliable source for load data. Powder charges are normally stated in grains, not grams. Nine to eleven grains of either FFg or FFFg will work.
Have you slugged the bore to determine the size bullet needed?