I agree with most of the above. ft. lbs. of energy doesn't kill, penetration and expansion does. But, while not perfect, the energy numbers are a rough guide to how much expansion and penetration you'll get.
All things equal, more energy = more penetration and expansion. That seems pretty accurate when you compare similar cartridges. If comparing 30-06, 270, etc I'd say it is pretty close. When you compare 223 to 45-70 or an arrow, the numbers can be misleading since they get their penetration differently.
The numbers I've always heard are 1000 for deer, 1500 for elk. Obviously it can be done with less, and there are many other factors to consider. But those numbers seem reasonable to me as a general guide.
