Nock down power is a myth. No bullet carries enough energy to the target to actually knock down a deer sized animal. A bullet kills by causing trauma to the vital organs. Sometimes a rifle will transfer enough energy if hit in the right spot to shock the nervous system and the deer will collapse from it. But you really didin't nock it off it's feet.
I shot a deer last year with a 600 grn slug at 20 ft. Probably a couple thousand ft/lbs of energy from that load at that distance. It punch right thru the deer and it ran 90 yards before it died. I took out the top of the heart and destroyed both lungs.
Here is an excellent link if you want to read all the information on it.
Terminal Ballistics
Paul