As others have stated it is based on velocity. The black powder granulation was 2F. In the 1909 Sears catalog they list their Pointer Smokeless loads as "Grains of smokeless powder equal to" X drams. You could have any load combination made up, and still only pay $7.25 per case of 500 shells.

Take a good read of the site HTH Bob mentions. It is one of the best on the net for this.

Pete