Recoil comes from the forces generated between the gun and the bullet upon being fired, and then from the gun and the exhaust gases as the bullet exits the end of the barrel. Imagine that you are the Normal force reacting to the force of the gun kicking back, you have to stand your ground so that the maximum energy to transferred into the bullet.
The equations that we use to find the force of the Recoil are pretty straight forward. We'll use 'g' for gun and 'b' for bullet. Being that the original momentums were zero, I have just left them out.
What you 'feel' from recoil can be mitigated to some degree, by making modifications to your firearm. Muzzle breaks disperse the gasses as they exit the end of the rifle, thus reducing the second recoil. Padding at the butt of the rifle or softer grips on a hand gun will absorb some of the energy that you would feel. These two things can make shooting large caliber rounds allot nicer.
There are also calculators you can use to figure your true Recoil.
