There is really no such thing as "knockdown" power. Tests on various dummies, as well as on people wearing bulletproof clothing pretty well show that no gun that can be held in the hands will knock a person down. A person who is shot falls down because he is seriously wounded or dead, not because some super-duper bullet knocked him down. Only in the movies, and with the help of a special harness, do people hit with an ordinary bullet go flying ten feet through the air.
A case in point - When the good Bishop-turned Confederate General Leonidas K. Polk, was sniped off Pine Mountain, Georgia, the projectile reportedly passed completely through his body but left him still standing. The weapon was a 20 pdr Parrott rifle, which was without a doubt, the largest sniper rifle used in any war. (I think it is legitimate to call it a sniper shot because it was deliberately aimed at a small group from a long distance, almost a mile.)
Jim