Originally Posted by
Doc Highwall
I am not sure that it is peening from the powder as much as I would believe that the powder was compressed against the base of the bullet upon ignition.
The powder when first ignited starts burning at the back of the case near the primer, pushing a wad of powder against the base of the bullet leaving a imprint on the base of the lead bullet, before all of it is ignited within an inch or less of travel down the bore.
To say that the powder peened the base of the bullet it would have to be traveling faster then the bullet like a sand blaster.
The powder and gasses cannot travel faster then the bullet while it is in the bore to peen the base of the bullet, but when the bullet leaves the muzzle the powder gasses can travel faster but only for a short distance.