It's been my (limited) experience that the powder that gives the highest velocity in a 22-24" length barrel will generally be the same powder that gives the highest velocity in a shorter 16-18" barrel.
It's my understanding that essentially all the powder that's going to burn will be burned within the first few inches in front of the chamber.
As the powder is burned it is converted to a gas. This gas then burns and expands until the available oxygen inside the barrel is consumed.
The fireball at the muzzle is the result of powder gasses reigniting as they come in contact with the atmosphere (oxygen), not powder continuing to burn and after exiting the barrel.
There may be a few granules of powder that didn't burn in the barrel but they're generally not a significant contributor to the fireball.
Exactly
