The shape of a mass affects its coefficient of drag. So a 1/480th of an ounce piece of lead in a thin plate shape will have much higher drag than a 1/480th of an ounce piece of lead in a tear drop shape. But both would have the same mass, inertia, and momentum, everything else being equal. If both left the muzzle at 1200 fps the plate shape would then slow down much faster than the tear drop shaped mass. Both had the same inertia before the shell was fired and the same momentum at the muzzle. After that the tear drop mass would maintain its velocity and momentum at higher values than the flat plate mass because its drag was less than the plate.
The shot at the bottom of the shot column are under more stress for the same reason the pressure at the bottom of the ocean is much higher than the pressure at the surface. Acceleration of the shot column taking the place of the earth's gravitational field in the ocean.
