Pressure waves in the body liquids created by a supersonic bullet wound can cause detrimental effects on cerebral functions and even break bones without touching them.

There was an interesting story in one of P.O. Ackley's books where he showed up at a burro cull where the other sharpshooters were amazed at how much quicker and cleaner the kills were with the 220 Swift as compared to the 30-06. These were 500-600 lb. animals.

Course, those guys weren't really interested in an exit wound like so many today that have to trail their game after a poor shot..

Col. Frank T. Chamberlain shot several hundred pigs, sheep, and goats while testing terminal ballistics in Puerto Rico after WWII. He confirmed that the supersonic bullets were much more efficient at killing than the slower projectiles. His research led the way for the 5.56 military round still used today.

.223 bullets do a fine job of killing these small Alabama deer at reasonable distances.