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Originally Posted by SC4006
...lets use for example .40 S&W. Why do the 155 grainers produce more felt recoil than maybe a 180?...
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There's a difference between recoil energy and felt recoil.
Recoil energy is a precise, physical quantity that is a function of the weight (mass) of the bullet, the weight (mass) of the powder charge, the muzzle velocity of the bullet, the muzzle velocity of the powder gases, and the weight (mass) of the gun. If you have those quantities for a given load in a given gun, you can calculate the amount of recoil energy produced when that load is fired in that gun. And a heavy bullet will produce more recoil energy than a lighter bullet fired from the same gun at a comparable velocity.
Felt recoil is a subjective matter. It's how you experience the recoil, and it's really something that only you can decide for yourself.
In general, for example, a lighter, fast bullet may produce recoil energy comparable to that of a heavier, slower bullet. But the recoil energy of the load with the lighter bullet will be manifest in a shorter pulse (distributed over a shorter period of time), while that with the heavier bullet will be distributed over a longer period of time. Depending on how you, personally experience these two types of recoil pulses, one may seem greater to you than the other, even though they really have similar recoil energy.
If you're interested, you can calculate the recoil energy of a given load using the following formula:
WG = Weight of gun in pounds
WB = Weight of bullet in grains
WP = Weight of powder charge in grains
VB = Muzzle velocity of bullet in f/s
I = Interim number (Recoil Impulse in lb/sec)
VG = Recoil velocity of gun (f/s)
EG = Recoil energy of gun (ft lb)
I = [(WB * VB) + (WP * 4000)] / 225218
VG = 32.2 * (I / WG)
EG = (WG * VG * VG) / 64.4
This formula is quite similar to a formula for free recoil set out at http://en.wikipedia.org/wiki/Free_recoil, although I think that the formula from Wikipedia may be a little more precise based on what I've read in Hatcher's Notebook. The formula I've reproduced above, is from the Q&As at http://www.frfrogspad.com/miscella.htm (specifically the question about why some guns of the same caliber kick harder than others). John Schaefer (FrFrog) notes that, "..."4000" is the nominal velocity of the powder gases at the muzzle for commercial smokeless powder and the observed range is between 3700 and 4300 f/s. It is sometimes stated as 4700 in some sources but this is based on observations of artillery, not small arms...." The Wikipedia formula would use the actual powder gas velocity, which may not be readily available.