All of the versions of Pyrodex (Select, P) that I have weighed all weighed 0,734 as much as a nominal BP. That is, 100 "grains" of Pyrodex from a BP powder measure set for 100 grains actually weighed 73,4 grains. Get same result for 777 FFG.
Actually, NO BP can be counted on to give the same weight as one might expect from the volume. They all normally weigh a bit more, Swiss normally does, or quite a bit less, as do Pyrodex and 777, or only somewhat less (Pinnacle). Check Cptn. George Baylor's webbsite for a full set of data.
Lars