The factors that determine the optimum "burning rate " (actually, the quickness) of the powder are all related to the specs of the cartridge as they are before the primer flash kicks the show onto the road � mainly, the net capacity of the case and the mass of the bullet. Less-than-optimum powders work after a fashion, but the optimum powders produce the highest velocities within the desired pressure limits with the case full (or nearly full) without having to be compressed.
The function of barrel length is to give the accelerating bullet the "run" space it needs to develop its eventual exit velocity. The "burning rate" of the powder is past history by this time.
Most charges of most powders (even the optimums) in most cartridges leave some kernels unburnt or incompletely burnt by the time the bullet exits the muzzle. The occurrence of unburnt or incompletely burnt kernels is not necessarily a matter of wasted powder. What count are the effects produced � the velocity and the pressure. Some of the best loads with available powders "waste" powder this way, if you insist that every kernel must burn completely.
There can be more than one "optimum" powder, because there are different categories of powder � single-base and double-base, to begin with. Also, the theoretically optimum powder for a given case and bullet may not exist � which may mean that more than one existing, available second-best powder may be the "best" as far as plain old country-boy practicality is concerned.
