Larry,
I am assuming that 7f and Null B are different granulations of more or less, the same thing. If so, mixed granulations will pack more densely that single granulation of either the larger or smaller sizes. Perhaps denser piles of powder slow the propagation of the burn.
I use 4f Goex, because I bought a pound of it for that express purpose - the deer die just fine, but I doubt the stuff is very fast.
With black powder the more surface area per unit of mass the faster the mass burns.
When you go from 1F down the burning ignition speed through a mass of powder increases. Until you go from 4F to 5F. The the ignition speed slows. The little air spaces between the grains of powder play a role in flame spreading rates through the mass of powder. But down around 5F the spaces become too small to allow flame spreading through the mass.
Dealing with Swiss Nul B.
The Swiss made black powder has a chemical rate of burn that is a good deal faster then other powders. In other words the rate at which individual grains burn from the surface to the core is faster with the Swiss powder than with any other brand. The Swiss powder having a sporting powder burn rate whereas GOEX and the others has a rifle powder burn rate.
In the flintlock pan the Swiss Nul B gives a more intense heat. Both in gas temperatures and how fast the heat is being produced by the burning powder. The NUl B is more efficient/effective because of its particular grain sizing and its faster chemical rate of burn.
In addition. The Swiss powder is produced with a high-purity potassium nitrate which makes it little effected by moisture in the air until the R.H. goes over 90%. When powder in a flintlock pan begins to pick up moisture from the air there will be a slowing of the flame spreading through the mass of powder.
E. Ogre
