How energy transfers from one object to another is very important to how it performs, and as we will see it makes the physics more complicated. 
    There are no specific formulas to explain how energy is transferred from one system to another.

Rotational motion

    A weapons recoil when firing is due to the fact that some of the energy of the explosion in the chamber is turned into rotational motion.

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     This rotational motion is because of the moving parts within the gun, and there relation to the center of mass. The entire body of the gun has a particular center of mass, As the weapon is fired the internal piston absorbs that force, and uses it to load another bullet into the chamber in a mechanical process.
    Because there parts a placed above of the center of mass of the hole system, they imparted some rotational motion into the system. The person firing the weapon experiences this as the force pushing upwards.
     Many manufacturers of have tried a verity of different things to reduce this. One method employed in the M16 assault rifle is to have two different pistons that work in different directions. This causes some of the rotational motion to cancel its self out.

  Surface Area and density of bullets

    The Surface area, shape and density of a bullet affect its stopping power.

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    Density of a bullet can be increased by adding a heavier materiel to the bullet. This changes the amount mass, and increases the amount of force the bullet impacts with.
    The surface area of the bullet effects how well the bullets penetrate its target. With a round flat tip, the bullet will come to a stop faster, imparting more of its energy sooner.
    A fine tip on a bullet will help cut through material faster. This is used to penetrate anything from a bulletproof vest, to a wall, or tank armor. This can have negative effects when impacting something soft, like flesh, because the bullet can go straight through its target, and fail to impart all of its energy.