Not quite, nature works on a path of least resistance model. Basically, nature is inherently lazy and wants to do as little as possible if it can help it.
Thus with shot, when the first plane hits and slows down, the second plane will hit the first plane, and seeing as these shot pellets are non magnetic and there is no force forcing them to clump together, will then begin sliding around (since that requires a lower impulse) the first set of pellets and slightly enlarging the depth of the first plane. The next plane will hit the second plane and so on and so forth.
The best analogy to shot is to think of it like a liquid. (1 molecule of water = 1 pellet of shot) There's quite a few quanta of shot, and each shot moves independent of another. There is no force that is holding them in a single rigid column. Thus, just like a stream of water, when it hits something solid fans out seeking the path of least resistance, so will shot. The path of least resistance is invariably to spread out side to side rather then push farther in and exert pressure on the already present, hard, and slow shot that has already stopped in place. Thus, once the first few planes penetrate as far as they will they form a rigid surface that the other pellets strike and skid sideways, producing a large shallow wound.
As for the rusted together pellets, I doubt they would do much better. Think of bonded/frangable rifled ammo. Sure it looks solid enough when its in the pressed state, but upon hitting a hard or semi-hard object it splinters back into its component powder and spreads out sideways in the path of least resistance. Think of it as #1000+ shot, since each grain of it can be thought of as an individual pellet. Thus, once again a wound is wide, shallow, and not overly effective.
