Hi Dave
Hopefully TAC will come along with a better answer than I am about to give - but I'm sure 777 is too fine for a rifle I know TAC also shoots Pyrodex & not BP.
I am using either TS2 BP or 777 in my Pietta & Ruger OA & use a 'medium' grade black powder (from Peter Starley) in my 3 band (experimenting with 65gn loads at the moment.
777 measures by vol - but with such a small grain - you are likely to get too much oomph IMHO
TAC where are youuuuuuuuuuuuuuuuuuuuuuuuuuuu ?
Roy
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