The grain size may well be close to the same for most BPs of various "F"s, but the power and density of the various powders today, just like in 1800, vary quite a bit. The only modern black powder I have had any luck with in regulating old double rifles has been Swiss, either 1-1/2Fg or FFg, usually the former. With shotguns I have not found the more expensive Swiss to be any better than Goex or Wano, but Swiss is tops for Black Powder Express doubles.
Sherman Bell (bless him!) has done a series of excellent articles in Double Gun Journal about regulating BPE doubles and the best powders for 'em, well worth finding! Also, Graeme Wright's book "Shooting the British Double Rifle" has an extensive write up of BP loading and techniques; I understand the third edition is coming soon.
