If a soldier shoots an enemy with the intent of killing him, he wants the bullet to deliver maximum energy. On the other hand, if he wants to basically knock someone down (say for dispersing a riot) he wants the bullet to deliver maximum force. To see that these are not the similar considerations, calculate the energy transferred to a very massive wooden block, and the average force exerted on the block when a bullet of mass 0.008 kg and a velocity of 400 m/smakes a collision of duration 6.0 ms. Think about a a rubber bullet that bounces back with no loss in speed (elastic collision) and an aluminum bullet which embeds itself in the block (Completely inelastic collision)