If anyone is still confused about this here is a quick explanation. Free recoil energy is the term used to describe the kinetic (or "moving") energy that is imparted by the rifle. The calculation does not take into account the shooter; it's as if the rife was somehow floating in midair when it was fired and free recoil energy is the result of that event. Your shoulder has to deal with this energy; the feeling of recoil is also affected by the impulse shape, but that is beyond the scope of our discussion. Calculating the free recoil energy gives us a way to compare various cartridges. Let's take three popular cartridges to get a handle on this. From the initial post we'll take the 7mm Rem Mag and the 308 then we'll add in the 7-08 just to solidify the example. The 7mm Rem Mag and the 7-08 are the key comparison, but the 308 will give us some more insight as well.
Let's assume all the rifles are the same weight, and for fun the same barrel length and number of groves and whatever else... Next we'll assume the bullets are going the same speed, and to that end we'll assume that the quickloads software calculations are accurate for the purposes of this example. (Disclaimer: these loads may not be safe in YOUR rifle. Don’t act like an idiot when reloading by taking info from some crazy guy on the interweb thingy)
Rifle weight: 8 pound rifle system
Barrel length: 24 inches
Bullet weight: 168 grain (note: the bullets used for this are all Berger hybrid 168gr in appropriate caliber)
Using the same powder in all of the cartridges eliminates that as a variable. It’s not the best or even appropriate for the different cartridges, but it works for this example. First let’s look at the 308. Before the ignition of the round the 308 consists of a case, primer, lump of powder and some air inside the case and a bullet. When initiated by the primer the powder and the air go through a process of deflagration which is a fancy way to say they burn slower than the sound speed of the material. It’s not a high explosive after all when the case is mostly filled by the powder. The powder burns and produces high pressure gas and that pushes on the bullet and moves it down the barrel. When the bullet exits the bore the gas is still about 5000 – 10000 psi. That gas ends up shooting out and around the bullet and even in front of it. All that gas started as a mass of powder and a mass of air, and that mass doesn’t go away when it burns; it’s just converted to a different material state; from solid to gas. And that gas is moving just like the bullet, so that’s why we have to take it into account when we calculate the recoil energy of the rifle. The gas has kinetic energy too! I used the quickloads software to calculate a load for the cartridges with my preferred powder Reloader 17.
For the 308:
(308, 47.75gr re17, 168gr hybrid, 2700fps, 24” barrel, 8lbs rifle)
The free recoil energy is 16.46 ft/lbs.
For the 7mm Rem Mag:
(7mmRemMag, 55.12gr, re17, 168gr hybrid, 2700fps, 24” barrel, 8lbs rifle)
The free recoil energy is 18.00 ft/lbs
And finally the 7-08:
(7-08, 44.21gr, re17, 168gr hybrid, 2700fps, 24” barrel, 8lbs rifle)
The free recoil energy is 15.74 ft/lbs.
So even though the 7mm bullets are both going the same velocity and are the same weight, the difference in powder charge makes the 7mmRemMag recoil about 14% more than the 7-08. The 308 recoils a little more than the 7-08, because the 308 requires more powder to get the job done. With a different powder this may not be the case, but whatever, this is just a theoretical exercise.
Also, a quick note on muzzle brakes… The reason they work is that they divert the gas sideways by putting a wall in front of it. The forward and backward forces fight against each other which reduces the recoil and the resulting lateral or radial forces cancel out. The above example does not include muzzle brakes. Fyi
Here is a nice recoil calculator:
Recoil Calculator
