The prime factor in the degree of hydrostatic shock imparted by a bullet (after, of course, its total momentum) is the degree to which that momentum is transferred to the body, and how quickly. The dum-dum
ammunition referred to above, and the hollowpoint
ammo that succeeded it, are designed to maximize shock value through maximizing surface area
When one of these bullets hits the body, it deforms upon impact. Hollowpoints and the softer dum-dums tend to mushroom outward when they hit. This causes the bullet to transfer more energy, since the larger surface area presented by the bullet after impact causes the amount of energy per square unit area to lower. This lower value means that the bullet has a tougher time penetrating (ripping into, tearing, etc.) the body, and instead transfers more momentum to the surrounding mass. This is usually referred to as 'stopping power.'
Higher stopping power leads to higher hydrostatic shock if the bullet deforms fast enough, which slows it more quickly. The shock is more damaging (the wave is higher pressure) the faster the energy is transferred.
One method of transferring more energy for higher stopping power other than the outlawed deforming ammo is to make the bullet fairly long, which induces it to tumble inside the target. The .223 round used in the American M-16 assault rifle does just that. Fully jacketed, the round rarely breaks up unless it hits a solid target like brick, stone, or metal; however, the tumbling action tends to not only transfer enough energy to provide the stopping power of a slightly larger bullet, but also produces fairly nasty 'tearing' wounds, which are even harder to deal with.