Knock Down Power

When I hear the term, "knock down power," my eye starts to twitch. If I am teaching a class, my first reaction is to ask the question, "and how exactly is knock down power measured?" Still no answers on that one.

Occasionally, someone will change over to the term "muzzle energy." But, since the initial term "knock down power" was used, chances are it was a debate - either 9mm vs. .45 acp, or .223 vs. .308.

Since nobody argues that .308 is a more powerful round than .223, let's focus on the main problem: 9mm vs. .45 acp.

Using ballistic calculators, as well as published data, muzzle energies of many common 9mm and .45 acp loads, data was gathered. 

On average, the 9mm rounds were right around 400 foot-pounds. The least powerful being around 390 foot pounds.

On average, the .45 acp rounds were right around 415 to 420 foot pounds.  The most powerful being about 450 foot pounds.

So, on average, there is a 20 foot-pound difference between the two rounds. 

When comparing the difference in pistol round muzzle energies to a lower powered rifle round, like .223, the numbers don't lie. 55 grain ammo has an average of around 1100 foot-pounds of muzzle energy.  So the difference between 9mm and .45 is roughly 2% of the muzzle energy of the common .223 rifle round.