The following is a simple for to allow GM and gearheads to quickly and easily calculate projectile weapons' Kinetic energy, free recoil energy and damage. The system for calculating damige is from T4's Fire, Fusion and Steel (2nd edition).
Free recoil is a simple and comparative measure of how much 'kick' is transmitted to the shooter. The GM may wish to take into account free recoil energy when determining if a weapon can be fired by a character of a particular strength, and where accurate follow up shots are possible.
Further Notes on Recoil
It should be noted that free recoil energy is really only a comparitive look at at recoil. Felt recoil is highly subjective and is effected by a large bumber of factors. The shape and area of the weapon/firer interface (butt) plays a large role. A larger butt area means less force is applied put cubic centimeter of contact area. Felt recoil can also be reduced through the use of recoil absorbing devices like rubber butt pads, recoil absorbing springs and in the case of CPR and ETC weapons, muzzle brakes. There are also tecniques of recoil redulction the use counter masses to absorb recoil energy like open bolt weapons and so called 'soft recoil' weapons, where a large percentage of the weapon is accelerated in opposition to recoil force, nullifying a large part of the recoil generated.
If one examined the force generated in firing use F=ma where m is the mass of the projectile an a is the acceleration of said projectile withing the weapon system, one can extimate the amout of recoil force generated, and translate that to the pressure applied to the butt over a given area to get an idea of the 'shock' delivered to the firer. A quick method to determine force is to assume constant acceleration of the projectil in the barrel. As force is equal in both directions (pushing the projectile to the target and the weapon to the shooter) we can reflate F to KE of the projectile by substituting F= m*v^2/2s where s is the barrel length in meters and assuming 0 for the initial velocity. But m*v^2/2 is KE, thus we arrive at F=KE/s in newtons (kgm/s^2) over the length of time the projectile accelerates down the barrel.