Here is a pretty good article about killing power:
Rifle Cartridge Killing Power List
By Chuck Hawks
I don't have much faith in killing power formulas in general. Most are obviously designed to reinforce someone's pre-conceived notions. I was curious to see what the results would look like if I included the most obvious, easily quantifiable, factors in a simple killing power formula. These factors are velocity, energy, bullet weight, sectional density (SD), and bullet cross-sectional area (frontal area).
After some thought and some time spent playing around with those numbers on my hand calculator, I found that if I took energy at 100 yards and multiplied it by bullet sectional density (a fractional number) and bullet cross-sectional area (also a fractional number), the result was a manageable two or three digit number, which I then rounded off to one decimal place. Ergo, the killing power number.
I choose energy at 100 yards because most CXP2 to CXP4 game is killed between 50 and 150 yards. 100 yards is right in the middle of that spread, and energy at 100 yards is included in most ballistics tables, so it's an easy number to use. Also, 100 meters is only slightly longer than 100 yards, so for all practical purposes the same killing power results will apply at 100 meters as well as 100 yards. If you want to compare the killing power of cartridges at some other distance, just plug-in the energy figures for that distance and work the equation.
For a more detailed description of the development of the killing power formula, please read "Rifle Cartridge Killing Power Formula" on the Tables, Charts and Lists Page.
Here is the formula:
Energy at 100 yards (in foot pounds) x Sectional Density (taken from reloading manuals) x Bullet Frontal Area (in square inches) = Killing Power figure at 100 yards.
The most important factor in killing power, by far, is bullet placement. The second most important factor is probably bullet terminal performance. The third most important factor is probably the physical and mental state of the game animal in question at the moment it is shot. This formula takes into account none of those factors. Unfortunately, those key factors are not quantifiable, at least by me. So, I am assuming that the hunter knows how to shoot, where to put the bullet, and that the bullet is appropriate for the game and conditions.
I will say that after I had initially calculated the results for a number of common rifle cartridges using the formula above, I was encouraged. The results seemed reasonable in light of my personal research and experience. I concluded that, although undoubtedly not perfect, these killing power numbers generally have a positive correlation with reality.
Remember that these numbers are the result of an attempt to apply a simple formula to an extremely complex problem. Unlike the creators of "pounds-feet," "impulse energy," "hydro-static shock" and other pseudo scientific terms, I want it to be clearly understood that these numbers do not represent any scientific quantity or unit of measurement. Use them as an indicator as seems appropriate, but do not attempt to make them into some sort of killing power dogma.
The list that follows is intended to suggest the relative killing power of various big game hunting cartridges and loads at 100 yards (or meters, if you prefer) when those cartridges are used appropriately. (Cartridge, bullet weight in grains, muzzle velocity in feet per second - killing power number at 100 yards.)
.223 WSSM (64 grain at 3600 fps) - 10.1
.243 Winchester (100 grain at 2960 fps) - 18.1
.243 WSSM (100 grain at 3100 fps) - 20.0
.25-06 Remington (120 grain at 2990 fps) - 26.5
6.5x55 SE (140 grain at 2700 fps) - 30.7
6.5mm Remington Magnum (120 grain at 3210 fps) - 30.2
.270 Winchester (130 grain at 3150 fps) - 35.0
.270 Winchester (150 grain at 2850 fps) - 37.4
.270 WSM (140 grain at 3125 fps) - 40.1
7mm-08 Remington (140 grain at 2860 fps) - 33.6
.280 Remington (140 grain at 3000 fps) - 37.1
7mm Remington Magnum (150 grain at 3110 fps) - 44.8
.30 Carbine (110 grain at 1990 fps) - 7.4
.30-30 Winchester (150 grain at 2390 fps) - 22.8
.30-30 Winchester (170 grain at 2200 fps) - 25.4
.300 Savage (150 grain at 2630 fps) - 30.0
.308 Winchester (150 grain at 2820 fps) - 34.7
.308 Winchester (180 grain at 2620 fps) - 46.2
.30-06 Springfield (150 grain at 2920 fps) - 37.3
.30-06 Springfield (180 grain at 2700 fps) - 49.2
.300 WSM (180 grain at 2960 fps) - 59.5
.300 Winchester Magnum (180 grain at 2960 fps) - 59.5
.300 Weatherby Magnum (180 grain at 3240 fps) - 72.8
7.62x39 Soviet (123 grain at 2365 fps) - 15.7
.303 British (180 grain at 2460 fps) - 40.1
.32 Winchester Special (170 at 2250 fps) - 25.4
8x57 JS Mauser (195 grain at 2550 fps) - 52.0
.325 WSM (200 grain at 2950 fps) - 75.6
.338x57 O'Connor (200 grain at 2400 fps) - 39.7
.338 Winchester Magnum (250 grain at 2650 fps) - 94.8
.357 Magnum (Rifle) (158 grain at 1830 fps) - 12.7
.35 Whelen (200 at 2675 fps) - 56.4
.350 Remington Magnum (200 grain at 2770 fps) - 60.9
.375 H&H Magnum (270 grain at 2690 fps) - 106.2
.416 Remington Magnum (400 grain at 2400 fps) - 188.4
.44 Remington Magnum (Rifle) (240 grain at 1760 fps) - 26.4
.444 Marlin (265 grain at 2325 fps) - 63.4
.45-70 Government (300 grain at 1810 fps) - 50.1
.45-70 Government (405 grain at 1330 fps) - 55.0
.450 Marlin (350 grain at 2100 fps) - 88.9
.458 Winchester Magnum (500 grain at 2090 fps) - 217.3