The reason I ask is that I have seen brass, usually well sooted in the neck/shoulder area, which I suspect didn't seal well right away so there was high pressure gas both inside the case as well as a very small amount outside the case around the shoulder. Obviously, if that high pressure gas is trapped even momentarily after the bullet exits the muzzle, the pressure inside the case can drop more rapidly while the high pressure gas is still trapped very briefly outside. Even a tiny volume of high pressure gas can increase greatly in volume and could cause a crease or dent like you see there. I would expect that there would be some soot thereabout however.
Last edited by Klikitarik; 04/21/15.
