The ignition of the powder creates a shockwave in the barrel that propagates up and down the barrel several times before the bullet reaches the muzzle.
When the shockwave is near the muzzle it creates vibrations that cause the tip to point in unpredictable directions.
When the shockwave is at the breech end the muzzle is relatively still and consistent. This is when you want the bullet to exit.
Loads that exit at the wrong time can be erratic and unpredictable.
Discussion of the theory is here.
Developing a load based on the theory, here.
Still happily answering to the call-sign Peetza.
You do not HAVE a soul. You ARE a soul. You HAVE a body.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.