Is Power Factor useful for comparing rifle calibers? For example, a typical 300 BLK PF (125 x 2,200 = 275,000) is quite a bit more than .223/5.56 (55 x 3000 = 165,000). Does that mean anything useful?

I personally put little stock in mathematical equations that are supposed to tell us how well a bullet will perform in flesh. Said formulas never take into account the type bullet being used, i.e.- FMJ vs. controlled expansion bullet.
Common sense should tell us that all things equal (bullet sectional density, construction, etc.), a larger diameter bullet will work better. But of course we also reach a point of diminishing returns because there's no such thing as "deader"

.. depending on exactly how you intend on using the data ??  Context ? Sure, of course it can be useful. It is part of the equation.

On the surface all it provides is a relative idea of kinetic energy available @ what ever range the numbers were run at...? Muzzle or POI ?  Or a starting point to run a ballistic program. Some of that E=MC/2 stuff.
It does not aid in Terminal Ballistics all that much unless a known type of projectile with known expansion properties is used. Which is where the rubber meets the road.
To make any useful comparisons you need Mass, Velocity, Bullet Coefficient, Bullet Construction, Expansion Thresholds. All while keeping in mind that Terminal Ballistics are often comprised by Exterior Balistics and the medium @ POI.

IPSC/USPSA's "Power Factor" is a measure of a projectile's momentum.  The units are non-standard but it is a measure of momentum, nonetheless, not energy.  Rankings and ratings using power factor would differ from the ranking using energy.  Momentum treats mass and velocity equally, whereas, energy greatly emphasizes velocity.  It does make a difference.

Momentum  = mass * velocity
Energy = 0.5 * mass * velocity^2

It applies to handgun and rifle cartridges alike.  I will show the difference for handgun cartridges because I am intimately familiar with their power factors.  These numbers are based on a 230 gr 45 ACP at 820 fps and a 125 gr 9mm at 1100 fps.

In my experience, momentum is more useful than energy for understanding a projectile's ability to knock down a steel plate.  IME, 45ACP is distinctly better than 9 mm at the plate machine.

Momentum (45ACP / 9mm) = 1.4
Energy (45ACP / 9mm) = 1.0

From a momentum perspective, 45 is 40% higher than 9mm.  From an energy perspective, they are equal.  Either will knock down a plate with a center hit but the 45 will knock it down even when you hit near the base or just nick an edge, the 9mm will not.  Cardboard targets won't know the difference but USPSA emphasizes good shot placement via the A zones on the targets (shot placement trumps momentum and energy in real life and in USPSA scoring).  

USPSA shooting is FUN!  

It sounds to me like you saw one of my posts in another section....maybe the AR15 variants subforum.

from what I can remember from physics,

PE = KE

potential energy = kinetic energy

the usual formula for KE is 1/2mv^2

the formula for PE is mgh

so you have mgh = 1/2mv^2

the masses cancel each other out, so you have

gh = 1/2v^2

where g is the acceleration due to gravity and h is the height that the object is.

in the real world, when something gets hit by a bullet, in my opinion, all you have is the mass of the bullet and its velocity.

you don't get hit by the velocity squared.

having shot plenty of USPSA and IDPA matches in the past, a major power factor pistol bullet (165,000) puts down the full size steel pepper popper targets with more authority than the minor power factor rounds (min of 125,000)

The flipside to the power factor didcussion is that I think the shooting competitions wanted to group similar recoiling handguns together.

Naturally, it is harder to shoot faster and accurately with a .45ACP versus a 9mm.

Or it usually is for newbies.  Once you get enough experience and practice in, the differences in split times and accuracy are neglible....I would think.

Well, actually, you do.  The projectile's momentum and energy are both manifest at the time you get hit.  Each "controls" a different effect in the terminal ballistics.  This is way off topic, so I'll stop.

Here are some numbers for 223 versus 30-06:

Momentum (30-06 / 223) = 2.4
Energy (30-06 / 223) = 2.1

So, yes, in momentum and energy, 30-06 has a huge advantage over 223.  Which would you take to Alaska to defend against Brown Bears?

I'd really rather not GD this thread up.

but again, in my opinion, you're not getting hit by 9,000,000 feet per second squared. (or per second per second)

and again, in my opinion,  how quickly a bullet can decelerate down to zero fps and still remain inside the target means it dumped all its "energy" into the target, and that is partly a function of the bullet's construction.

in my opinion, a bullet exiting a target is wasted "energy", but there might be obvious reasons that you'd want a bullet to pass clean through.

touting that such and such super duper ammo has 5 gazillion foot pounds of energy is, again, in my opinion, just a marketing schtick.

I doubt you will ever see Winchester or Remington put power factor labels on the outside of their ammo boxes.

and supposedly, anecdotally, the native people of Alaska love their .223s or .243s for polar bear hunting.

supposedly....

I am just trying to present some technically correct info (momentum and energy).  I am not intending to get into terminal ballistics, just using some practical examples for comparison.  

223 for polar bear, eh?  Well, like I said, shot placement trumps momentum and energy.  But you know that.  

Shot placement trumps all.  


The BIG jump is a nervous system response to being stuck with a pin.  It has nothing to do with energy or momentum.  

I would say this thread has gone completely into General Discussion territory.  I am out of here.

Most folks use ft lbs (of energy) when comparing rifle caliber IME.

Also commonly used, especially by African hunters, is  John 'Pondoro' Taylor's formula to calculate  his 'Knock Out' values which take momentum and throws bullet diameter into the mix

KO= m*v/bullet dia in inches

How so?

OP asked a question and folks are answering.

Just because it is a tech forum doesn't mean we have to agree.

You just need to present your disagreement in an adult, logical, polite manner.

Now, back to OP's question, yes you can debate the merits of power factor for rifles, but IME, it is not commonly used. Taylor's KO is the closest use of momentum in rifle discussions with which I am familiar.

As was mentioned, formulas always fall short when comparing apples and oranges because they are, after all, apples and oranges. 

For example, some rifles just seem to kill better (anecdotally) in the field than their ballistics would suggest.  Calibers that come to mind are the 30-30, 7x57 Mauser, and the 375 H&H.

Physics is natural law without emotion or prejudice.  It is the application of that law which is subject to discussion, speculation and desire.  I think most folks pick a methodology that reinforces their prejudices regarding caliber, bullet weight, bullet construction, and velocity instead of the other way around.

The destruction they produce on two legged things is very different.

With a .223 you may have to amputate an appendage due to tissue destruction.

With a .30-06 is might already be gone.