Actually, regarding smokeless powders, irrespective ofa powder's "burning rate", the great majority of powder charges in modern, CF rifle cartridges are completely consumed by the time bullets have travelled four to six inches down the barrel. (Long before the bullet reaches the muzzle!!) Therefore, the powder charge that will produce the highest velocity is the one that consists of the greatest mass of powder which can be burned in a given cartridge without generating excessive pressures. (ie,the most fuel. )

This is usually the slowest-burning powder that will produce the maximumPERMISSIBLEpressure in that gun with the bullet in question. NOTE THAT THIS IS TOTALLY INDEPENDENT of the length of the barrel!

Of course, the longer that pressure can push on the bullet, the higher the velocity will be when the bullet leaves the muzzle, so, yes, barrel length has an effect. BUT, under normal circumstances, NO POWDER is still burning when the bullet exits the muzzle, unless the powder was loaded to a density insufficient to permit normal burning - (too little powder....)

For more definitive info on this subject, I refer you to this months' HANDLOADER Magazine. Read about Phil Sharpe's actual experiments on barrel length vs MV..... It is pretty eye-opening for most people!