In my opinion, most of what people describe as "knockdown power" is a bunch of hooey. Even a .50 BMG will not knock a buck completely off its feet. That being said, you can shoot deer (or other big game) in the right spot with the right bullet and kill it where it stands. "That" type of kill is what I believe most people refer to a "knockdown power."

Any decent bullet that enters the brain or severs the spine will kill a game animal pretty much instantaneously. A bullet through both lungs tends to down them with 25-50 yards, while a heart-shot animal can run from 75-100 yards or more. Lung and heart shots kill by destruction of tissue and blood loss, and a ragged hole through both lungs bleeds more profusely than a hole in the heart. Sounds crazy, but the heart is a strong muscle, and a bullet hole will open and close as the heart beats, while the lung tissue will spurt blood like a garden hose.

That is why a bullet that passes through the aorta and surrounding arteries/veins is a great shot. If the aorta is destroyed, the entire oxygenated blood supply enters into the body cavity. Once this occurs, the brain will rapidly be depleted of oxygen and the animal will fall unconscious and die quickly.

All that being said, the quickest kills I have made on deer have been shot with my .30-06 and 165-grain Nosler Ballistic Tips. In almost every case the bullet has completely destroyed either one or both lungs, the heart or the spine. None of the deer so shot made an additional step after receiving the bullet. I will agree that if a game animal is totally unaware of the hunter, a bullet that causes less damage (say a Winchester Fail-Safe, that expands less but penetrates deeper) may drop at the shot, but if it is alerted and wary it may still run off, although dead on its feet. But when both lungs and the pipes above the heart are wrecked beyong comprehension they normally do not make it farther than ten yards.

Now, knockdown power should not be confused with stopping power, especially in the arena of dangerous game. You can kill a 550 lb lion with a .250 Savage, but I wouldn't want to try and stop a wounded Simba determined to masticate me to death with one. You need big, heavy, well-constructed bullets capable of smashing heavy bone and penetrating deeply in order to have a reliable stopping rifle. Normally you could say this starts with a 250 grain slug in a .338 Win Mag and ends at a 900 grain .600 Nitro Express.

At least that's how I see it.

Brian