I have only had 2 deer drop on the spot. I have shot them with about everything from a 6.5x55 thru 35 Whelen. Unless the brain or spinal cord it hit, there is NO guarantee that they will drop, irregardless of what you shoot.
Not to mention "knockdown power" is a bit arbitrary. Foot pounds doesn't impress critters. Destruction of tissue, vital organs and a loss of blood and oxygen are what kill (outside of the aforementioned brain pan, and spinal shots).