Almost ALL the powder that's going to burn, will burn within a couple inches in front of the chamber. The same powders that result in the highest velocity in a 24-26" barrel will result in the highest muzzle velocity in a 20-22" barrel.
The "flash" at the muzzle isn't powder that's still burning, but the hot gas reigniting when it hits the atmosphere's oxygen. Rifle powder contains it's own oxygen source, but by the time it leaves the muzzle is oxygen-starved. It's still plenty hot, however, and with the "new" source of oxygen reignites--but isn't likely to cause a forest fire, unless you insert the muzzle into a bunch of dry tinder.
