I would tend to agree with 8-Bore on the powder granulation. Powder burn rates are based upon load density & velocity, so for black powder shotshells this gives load density as almost the exclusive factor, unless some "Extreme" loads are used. Load density is the wt of charge to be moved in proportion to area (sq-in) of bore. Of all standard shotshell loads the 1-1 1/8oz 12ga & 1 1/8-1¼oz 10ga loads generally speaking have the lowest densities of any. FFFG in mild loads is very successful in these loads. If shot charges heavier than these are used FFG should normally be switched to & even FG if very heavy loads are used. Smaller ga's than 12 were mostly loaded to a higher density in order to obtain a better shot charge wt & also ga's larger than 10 were normally loaded to higher densities as these were generally loaded with large shot for an attemt at very long range use so very heavy charges were common.
The 5½ x 1 3/4 load 8-bore mentions for the 8ga would "In My Opinion" be about what I would consider as heavy as I would want to go with FFG & like him would probably prefer FG, for that or heavier.
It should in fairness however be noted that 10ga is the largest size shotgun I have ever had opportunity of firing with either Black or Smokeless.
