Ft-lbs is a mathematical way to measure energy. It is one way to attempt to predict how effective different ammo would perform on live targets, either human or animals being hunted.
It should be a rough guide only. A cartridge that delivers 700 ft-lbs isn't necessarily any better than one that only delivers 300 ft-lbs. Destroyed body parts are what kills stuff. Sometimes a bullet with more energy will destroy more body parts than a bullet with less energy, but not always. What is much harder to predict, but is far more important, is penetration, bullet construction and expansion and bullet placement.
Often a smaller diameter bullet, with less energy will out penetrate a larger diameter bullet with far more energy because the energy is concentrated in a smaller area. Or because of differences in bullet construction, (FMJ vs HP). A 22 rimfire with fmj ammo will usually out penetrate many much more powerful rounds.
You also have to consider just how much energy is really needed. In a hunting rifle, you don't need a lot of power to kill a whitetail deer. Bigger, more powerful rounds don't kill them any quicker, or better than most of the less powerful rounds.
