When put into its simplest terms, guns are not very complex machines. The goal is to deliver the maximum destructive energy with the minimum amount of energy to go to the shooter. Better more expensive guns will do a better job at accomplishing this task. However, conservation of momentum is true in guns as well. That means that the momentum delivered to the target still has to be equal to the momentum felt by the shooter. This is felt through the recoil of the gun. That is why bigger more powerful guns will have a bigger kickback on the person shooting it. The goal of a firearm is also to use energy efficiently and that is dependent on its construction. The two main factors to this energy efficiency are the caliber of the gun and it's barrel length.