AB, I think what 481 was saying that while energy goes up in direct proportion with an increase in mass, it goes up as the square of a velocity increase.
Twice the mass, twice the energy. Twice the velocity, four times the energy. So, if energy is the overriding concern, the answer will typically be to increase velocity, rather than mass.
Momentum goes up in linear fashion with both mass and energy.
The resistance of a medium to a bullet will be a function of energy. Look at what happens, for instance, when fast moving objects hit the water.
The momentum of the bullet is what will fight the drag from the medium. The slower, heavier bullet will have the advantage.
