Thread: The Maths of Recoil

Originally Posted by grandpamac

Greetings Micky Duck and others that may be interested,
Sorry Micky I had not answered your question so now, fortified with a night of slumber, a bowl of muesli and a large cup of tea, I will attempt to address it a bit better.
First I looked at the powder charge needed to produce the 2,500 fps velocity. The Hodgdon data for AR2208 and the .303 with the 150 grain projectile. Next I looked at velocity loss from the 24 inch test barrel to 14 inch. Using the lower end of the often quoted 20 to 25 fps per inch this works out to a loss of about 200 fps. The increase of velocity form 24 inches to 30 inches will be less so lets say a gain of 50 fps. AR2208 needs about 1 grain more powder to increase velocity by 50 fps in a 24 inch barrel so to achieve 2,500 fps the 14 inch barrel needs 44 grains, the 24 inch 40 grains and to 30 inch only 39 grains.
Now to recoil energy. The recoil generated by the projectile is the same for each barrel length. The velocity of the powder mass in high power rifles was given as 1.75 times projectile velocity. This is a rough average and the longer barrel will deliver less and the shorter one more. The formula for energy is 0.5 times mass times muzzle velocity squared so even the same powder charge will produce greater recoil in the shorter barrel than either of the longer barrels. The extra powder needed to achieve the same velocity in the shorter barrel makes this even worse.
So should we all ditch our shorty barrels and screw in 30 inch ones? Probably not. But we do need to remember that short barrels come at a cost, both velocity and recoil.
Regards Grandpamac.

That is a very clear way of explaining it GPM and would have Sir Isaac thinking. What interests me is if I am going to build a lightweight rifle in 6.5 mm that achieves 3000 fps with a 100 grain Barnes and have clean slate to start with, which calibre do I go with?. As an example only, I could go with a 6.5-284 and load it down or a 6.5 x 47 with a normal load. The 6.5 x 284 will need more powder and create more recoil to reach the same velocity with the 100 gr Barnes.