“...So the ar-15 packs a punch in energy...” [rlbedfor, post 58]
“...With the right barrel twist, bullet, load and a fairly accurate hit, it will take a deer right down. It’s legal for deer hunting in some states...” [familyop, post 59]
Kinetic energy is the best measure of small arms cartridge/projectile lethality, as determined by US Air Force Research Laboratories. The key word here is “best:” as with most weapon effects, actual outcomes are highly probabilistic.
E = (M*V*V)/450380
will give you kinetic energy in foot-pounds, where M is bullet weight in grains, V is velocity in feet per second, and the four-digit number is the required conversion factor.
As familyop noted, a very great deal depends on projectile shape & material, and just what it hits on striking a living target.
In the state of New York, in 1970, any centerfire rifle was legal for taking deer (in those counties that did not mandate the use of shotguns).
In 1977, in South Dakota, I watched a buddy take down a pronghorn antelope with a single round from a 222 Remington. Slightly less kinetic energy than 223 Remington; 5.56x45mm was developed from 222.