If you take this F=ma equation you can figure out how much force is generated from an object of a known weight accelerating at a known rate. Acceleration (a) is basically a change in velocity (v) over a change in time (t). The F=ma equation will give you the amount of force being applied during the acceleration. After the object stops accelerating you can calculate the amount of kinetic energy imparted to it through acceleration by the equation KE=mv^2 since (a) in the previous equation would go to zero. The KE equation is going to determine how powerful the bullet is, the F=ma equation will determine how much recoil it is going to generate getting up to speed. Remember this snippet, we will discuss it in the next section. Newton's Third Law states: To every action there is always opposed an equal reaction; or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts. This law is more commonly paraphrased "For every action there is an opposite and equal reaction". So if we accelerate a bullet forward, then the gun has to accelerate rearward. When you pull the trigger and the propellant ignites, it expands and pushes the bullet forward while at the same time pushing the gun back. Therefore, the force of the bullet's acceleration is going to be equal to the force of the gun's acceleration (recoil). Lets put this in an equation format: F=ma ----->ma [rifle]=ma [bullet] so therefore F [rifle]-F[bullet]=0 Basically the gun and the bullet are going to have the same force exerted on them but the bullet will accelerate to a higher speed because it is lighter. If the bullet gets heavier/faster or the gun gets lighter, then the gun will accelerate more (kick harder).