Oldmanofthesea,
My understanding from various readings is that by the time the bullet exits, the expanding powder gas is usually pretty oxygen-starved--which causes the HOT gas to reignite due to the oxygen in the atmosphere--which is what causes muzzle flash, NOT still-burning powder granules, as so many assume.
Also, if the powder charge produces enough pressure to be within the general design parameters of that powder, then all (or almost all) of the powder will have burned within a few inches of the chamber. If the pressure never gets that high, then some of the kernels won't burn, often a LOT of 'em. But accuracy can still be excellent, as I've found with various low-pressure loads over the years using easily-ignited powders.
It's when harder to ignite powders don't quite reach the "design pressure" that accuracy tends to suffer--one reason the accuracy of "starter loads" often has no resemblance to accuracy at max pressures.
